What are the defences to a drink driving charge?

1. Drink Driving Laws and Defences

Drink Driving in the UK

Primary Legislation: The principal statute governing “drink driving” in England and Wales is the Road Traffic Act 1988 (RTA 1988). Under Section 5(1)(a) RTA 1988, it is an offence to drive or attempt to drive a motor vehicle on a road or public place with alcohol in one’s system above the prescribed legal limit​

Drink driving is sometimes referred to as “driving with excess alcohol”  or a "DUI" and is a strict liability offence – the prosecution need only prove the alcohol level exceeded the limit at the time of driving, not that the driver was impaired or intended to drive drunk​. It does not matter if you had no intention to drink driving or if it was accidental. The prescribed alcohol limits are defined in law (35µg of alcohol per 100ml of breath, 80mg per 100ml of blood, or 107mg per 100ml of urine)​. The amount of alcohol required to place you above the legal limit in England and Wales may actually surprise you.

Drink Driving Calculation Explained...

If an average-height, average-weight male consumes four pints of mid-strength lager between 6 PM and 11 PM, he will have ingested a total of 109.06 ml of ethanol (alcohol). By 11 PM, after accounting for the body's natural elimination process, approximately 59.06 ml of alcohol would remain in his system. This equates to 35.4 micrograms (µg) of alcohol per 100 milliliters of breath. Since the police typically do not prosecute unless the breath alcohol level exceeds 40 µg, this individual would technically be under the limit at this point.

However, if he were to consume an additional pint of beer within the same timeframe, his breath alcohol concentration at 11 PM would rise to 51.6 µg, exceeding the legal threshold. That said, by the time he decides to drive and undergoes a breath test, his alcohol level may have decreased enough to fall below the legal limit due to continued alcohol metabolism.

Drunk "In Charge"

Being “In Charge”: The law also criminalises the act of being “in charge” or "in control" of a vehicle while over the limit (RTA 1988, s.5(1)(b)). This covers situations where someone is not actually driving but may intend to. The most common example is falling asleep in the vehicle. However, the statute provides a specific defence for this scenario. Section 5(2) RTA 1988 states that a person in charge of a vehicle over the limit is not guilty if they can prove that “the circumstances were such that there was no likelihood of [them] driving the vehicle while [over the limit]”​. This places a legal burden on the defendant (on balance of probabilities) to show they had no intention to drive again until sober – for example, someone sleeping in their car with no realistic prospect of resuming driving may invoke this defence. To successfully argue this defence, the court would normally require;

1. A plausible explanation as to why you were drunk in charge
2. 'Witnesses' to confirm that you would not have driven (i.e. a friend who can confirm that he was en route to pick you up)
3. A BAC Calculation report showing when your alcohol level would have fallen below the limit
4. Evidence that you might have called a taxi 

The Section 15(2) presumption

It might seem unfair, but the police do not have to prove that you were over the legal alcohol limit at the exact time you were driving. Instead, they only need to show that you were over the limit when you took the official breath test at the police station.

This can be a major issue for someone who drank alcohol just before driving. At the time they got behind the wheel, their blood alcohol level may have been within the legal limit, but as their body absorbed the alcohol, they became over the limit by the time of the breath test.

Another scenario is drinking after driving—sometimes called the "hip flask defence." This happens when someone drinks alcohol after an accident or incident but before the police test them. They may argue that they were sober while driving and only went over the limit because of what they drank afterward.

However, under Section 15(2) of the Road Traffic Offenders Act 1988, the law assumes that if you were over the limit at the police station, you were also over the limit when you were driving. This means the burden is on you to prove otherwise.

Fair or not? Let’s take a closer look at the legal framework behind this rule.

Legal Framework: Road Traffic Offenders Act 1988

The Road Traffic Offenders Act 1988 (RTOA 1988) contains a number of rules for drink-driving cases. Specifically, Section 15(2) of the RTOA 1988 establishes the presumption that the alcohol level in your breath, blood, or urine at the time of the test was at least as high as it was when you were actually driving.

In other words, the law assumes the body alcohol level at driving was at least as high as when later tested. This presumption greatly aids the prosecution by linking the evidential specimen to the time of driving. However, as discussed below, it can be rebutted by the defendant in certain circumstances (i.e. the “hip flask” defence).

Note: One and sometimes 'easy' defence relates to the requirement for the police to provide proof of alcohol levels by documentary evidence. Section 16 requires the police to serve upon the defendant a copy of the breath test printout at least 7 days before trial. A failure to do so may bar the CPS from relying on the printout and therefore proving that you were over the limit.

Other Applicable Rules: Drink driving investigations are subject to specific procedures. The police must follow the Road Traffic Act procedures for obtaining specimens (RTA 1988, s.7) – typically requiring two breath samples on an approved device and taking the lower reading as evidence​. The devices used (e.g. the Intoxilyzer or similar) are type-approved by the Secretary of State, and courts generally presume such devices function correctly unless shown otherwise. ​In addition, the Police must also give a statutory warning to the suspect about the requirement to provide specimens and the consequences of refusal​. These procedural rules, mostly set out in the RTA 1988 and detailed in guidance (the MGDDA – Manual of Guidance for Drink and Drug Driving), form part of the legal framework and, as we will see, failures in procedure can provide defences.

Finally, it should be noted that drink driving offences are usually tried in the Magistrates’ Court (they are summary offences, punishable by up to 6 months imprisonment and a mandatory driving disqualification). If convicted, the offender faces at least a 12-month driving ban in a first offence​. Given the serious consequences, the law recognises a number of defences and safeguards, discussed below.

2. Defences to Drink Driving Charges

Several defences are available to a motorist accused of drink driving. These defences range from statutory defencesprovided in the legislation to general criminal law defences and technical challenges to the prosecution evidence. Below is a breakdown of the main defences, including those specifically mentioned in the query (the “hip flask” defence, necessity/duress, procedural impropriety, lack of evidence/unreliable readings, and medical issues):

(a) Post-Driving Alcohol Consumption – The “Hip Flask” Defence

One of the most well-known defences is the “hip flask” defence (also called post-incident consumption or rising blood alcohol defence). This applies where the driver admits drinking alcohol, but only after the driving took place, such that their alcohol level was below the legal limit during driving and only rose above the limit later. In practical terms, a defendant might claim they were sober (or under the limit) while driving, but then consumed alcohol (for example, from a hip flask, or drinks at home) before the police administered the breathalyzer. The result is a high reading at the police station that does not reflect the level at the time of driving.​

The hip flask defence is expressly acknowledged by the law. RTOA 1988 Section 15(3) provides that the usual presumption about the alcohol level at the time of driving (in s.15(2)) “is not established if the defendant consumed alcohol after he had ceased to drive (or be in charge) and before providing the specimen”, so long as it’s shown that without that post-driving alcohol, the proportion of alcohol in his body would not have exceeded the limit (and would not have made him unfit to drive)​. In essence, the defendant must prove three key elements:

1. that they drank alcohol after driving and before the specimen was taken,
2. that they had finished driving (or being in charge on a road) before this post-drive drinking occurred, and
3. that, had they not had those post-driving drinks, their alcohol level at the time of the test would have been within legal limits (and they would not have been impaired)​

If these elements are proven, the statutory presumption is rebutted and the defendant should be acquitted of the over-limit charge.

Burden of Proof: Notably, the hip flask defence imposes a reverse burden on the defendant. Unlike most criminal defences where the prosecution must disprove the defence, here the law requires the defendant to prove on the balance of probabilities that their post-driving alcohol caused the excess reading​. This means the driver must persuade the court it is more likely than not that they were under the limit while driving and only went over due to drinking afterwards. In practice, this often requires expert evidence (for example, a forensic toxicologist performing a back calculation of what the driver’s blood alcohol concentration would have been at the time of driving based on the timing and amount of post-incident drinks)​. Other evidence like witness testimony or CCTV showing the driver was sober prior to the incident can also support the claim​.

If the court accepts the evidence and finds it more likely than not that the post-driving drinks pushed the defendant over the limit, the defendant is entitled to be found not guilty of the drink driving charge​

Case law has tested this defence. For example, in Parker v DPP [2001] RTR 240, the validity of placing this burden on the defendant was challenged under human rights law – but the High Court upheld the law, noting that requiring the accused to rebut the presumption is reasonable and compatible with the presumption of innocence given the public safety interests at stake​. Courts have also made clear that if a defendant presents a credible hip flask scenario with supporting evidence, the magistrates must assess it carefully. In Dawson v Lunn [1986] RTR 234 (an earlier case under prior legislation), and Cracknell v Willis [1988] 1 AC 450, it was affirmed that a driver can challenge the assumption about their alcohol level by providing evidence of post-driving consumption​

The ultimate question is whether the court is satisfied beyond reasonable doubt that the defendant was over the limit at the time of driving – if the defendant’s evidence raises a doubt (or on balance shows they were likely under the limit while driving), they should be acquitted​

It should be stressed that merely asserting a hip flask defence is not enough; it “is not necessarily successful”, especially if the story is not believed or if the scientific analysis doesn’t support it​. Courts are understandably cautious – this defence has a bit of a reputation for being an excuse of convenience, so strong proof is needed. But when properly evidenced, it can and does succeed (for instance, where an expert’s back-calculation corroborates the defendant’s account, it can be “difficult for the prosecution to refute”​).

(b) Necessity or Duress of Circumstances

Another potential defence is necessity (also known as “duress of circumstances” when involving a threat of danger). This is a general defence that can apply to any criminal charge if the defendant was effectively forced by circumstances to break the law in order to avoid a greater harm. In the context of drink driving, this might arise in scenarios such as: driving while over the limit to escape an imminent threat (for example, fleeing from an attacker), or driving in an emergency (such as taking a gravely ill person to the hospital) despite being over the limit. If the defendant can show they only drove because of a necessity to prevent death or serious injury, courts may accept this as a complete defence – essentially that the illegal driving was justified by the emergency.

Historically, English law was very reluctant to allow necessity as a defence to driving offences. A classic example is R v Kitson (1955), where a man found himself in a runaway car while intoxicated (he wasn’t driving initially, the car was coasting down a hill driverless). He grabbed the wheel to steer to safety, yet he was still convicted of drunk driving. At that time, the courts did not recognise necessity in such circumstances. However, the law evolved in the late 20th century. By the 1980s, courts began to acknowledge duress of circumstances as a legitimate defence for driving offences. In R v Martin [1989], the Court of Appeal held that the defendant should have been allowed to plead duress of circumstances when he drove (while disqualified) because his wife threatened to commit suicide if he didn’t drive their son to work​. The appeal court accepted that a threat of serious injury or death (even suicide of a loved one) could create a necessity that excuses the driving offence. Following Martin and similar cases (e.g. R v Conway [1988] for dangerous driving), it is now established that “duress of circumstances” can apply to driving offences, including drink driving, if the facts warrant it.

To succeed with this defence, the situation must meet strict criteria. Generally, the driver must show that they reasonably believed they or others faced an imminent danger, that driving was absolutely necessary to avoid that danger, and that there was no reasonable alternative. The response (driving while over the limit) must be proportionate to the threat. For example, a person who, while over the limit, drives someone in a medical emergency to the A&E may invoke necessity – especially if calling an ambulance or other options were impractical. Likewise, someone fleeing a physical threat might claim they had no choice but to drive away despite being intoxicated. If the court finds this credible, the defendant may be acquitted on the basis that a reasonable person would have done the same in that crisis.

It should be noted that duress/necessity in drink driving cases is rare and not easy to establish​. The courts will closely scrutinise such claims to ensure they are not abused. Also, threats of a lesser harm (like losing one’s job or minor injuries) would not suffice – the defence is reserved for grave circumstances (typically risk of death or serious injury). Importantly, self-induced necessity is not a defence (for instance, one cannot deliberately get intoxicated and then claim an emergency forced them to drive). But genuine, unforeseen emergencies can justify what would otherwise be an offence.

If a defendant intends to rely on duress of circumstances, this defence would be raised at trial by presenting evidence of the threatening circumstances. Often the defendant’s own testimony is crucial, and possibly any corroborating evidence of the emergency (witnesses, medical records, etc.). If the court (or jury) accepts the defence, the result is a full acquittal. As an aside, even if the strict requirements of the defence are not met, such circumstances might be raised as “special reasons” to avoid a mandatory disqualification (discussed later), but that is a mitigation argument rather than a true defence.

(c) Errors in Law Enforcement Procedure

Procedural mistakes by the police can provide a powerful defence or bar to conviction in drink driving cases. Because drink driving procedure is heavily regulated, failure to follow the prescribed steps can render the evidence inadmissible or unreliable. If the police “deviate from the set procedure in any way, the case against you must fail.”​ This is a strong statement, but it reflects the fact that the breath/blood testing process has legal safeguards which, if breached, can undermine the prosecution.

Some examples of procedural errors include:

• Failure to give the Statutory Warning: Before requiring an evidential breath test at the station, the officer mustinform the suspect of their obligations – typically by reading the statutory formula: “I require you to provide two specimens of breath for analysis by means of an approved device… I warn you that failure to provide either specimen will render you liable to prosecution.”​

  If this warning is not given at the proper time, any subsequent test might be challengeable. The absence of the warning can mean the suspect was not made aware of the legal requirement, potentially providing a defence for failing to provide a specimen or calling into question the lawfulness of the procedure​. In practice, courts have held that an omission of the warning or a mistake in delivering it can justify excluding the evidence or even dismissing the charge, since the statutory precondition wasn’t met.

• Lack of Consent for Blood/Urine Tests: If the breathalyzer is unavailable or the suspect can’t blow properly, the police may require a blood or urine specimen. In such cases, consent and proper procedure are critical. The suspect should be asked to consent (because taking blood or urine implicates bodily integrity). If an officer or medical practitioner fails to obtain valid consent before taking a blood sample, that sample might be ruled inadmissible​. Additionally, the suspect must be given their own portion of the blood sample to independently test if they wish. Not offering the defendant a part of the blood specimen (for private analysis) is a procedural breach that can invalidate the evidence – since the law intends to give the accused the opportunity to verify the result independently.

• Improper Administration of Breath Test: The Home Office approved devices require two breath specimens. Only two attempts should be taken (absent machine error), and the lower reading of the two is used​. If more than two evidential breath samples were taken (e.g. an officer keeps taking additional readings beyond the legal two attempts), this can be a technical defence – the extra tests are not authorized, and any result obtained in breach of the two-sample rule may be inadmissible​.

  Similarly, if a blood sample is taken, it must be done in one go (though it can be split into two vials afterwards). The police cannot, for instance, take two separate blood draws and combine them to get a sufficient quantity – doing so would violate the one-sample rule​.

• Observation Period and Mouth Alcohol: Police procedures (and the MGDDA guidelines) require that before an evidential breath test, the suspect should be observed for a 20-minute period to ensure they haven’t eaten, vomited, or placed anything in their mouth (which could contaminate the sample with residual alcohol). If, say, the suspect regurgitated or used mouthwash containing alcohol and the officer didn’t notice or restart the observation period, the breath reading might be tainted by mouth alcohol. Failure to adhere to this protocol could be raised by the defence to cast doubt on the breath reading’s accuracy. (This ties into “medical conditions” discussed below – e.g. acid reflux causing mouth alcohol.)

• Right to Legal Advice: There is no absolute right to consult a lawyer before providing an evidential specimen (courts have held that securing the breath/blood sample takes priority over immediate legal advice). However, if police improperly prevent a suspect from communicating or if they delay the test unreasonably while advice is sought, complex arguments might arise. (In general, though, the courts have rejected the idea that insisting on a lawyer before breath testing gives a “reasonable excuse” for not providing a sample​.)

• Disclosure and Evidence Handling: The Crown Prosecution Service (CPS) must disclose the evidence they will rely on in a timely manner pre-trial. If the prosecution fails to serve key evidence by the disclosure deadlines, the defence can object and apply to exclude that evidence​

• In fact, late or non-disclosure of, say, the breathalyzer printout or analyst’s certificate can “shut down” the prosecution case entirely​. For example, if the CPS hasn’t provided the breath test printout or laboratory report to the defence at least 7 days before trial (as required by RTOA 1988, s.16(3)), the court may refuse to admit that document​. Without evidence of the blood-alcohol level, the prosecution cannot prove the charge, so the case might be dismissed​.

In summary, procedural errors can be raised as defences typically by way of a pre-trial motion or during the trial in the form of a submission of “no case to answer” (if critical evidence is inadmissible). Defence solicitors often scrutinize the MGDDA procedure forms the police fill out, looking for any deviation. It has been said that the drink-drive procedure is “extremely technical and complex” and requires strict compliance​. Courts will exclude evidence under Police and Criminal Evidence Act 1984, s.78 if admitting it would have an adverse effect on the fairness of proceedings – a breached procedure can trigger such fairness concerns. A notable illustration was DPP v McKeown & Jones (1997), where a purely technical issue (the machine’s internal clock was wrong by over an hour) initially led a court to overturn convictions; the House of Lords later reinstated them, emphasizing that since the timing error did not prejudice the accuracy of the result, it did not invalidate the process​.

This shows that courts distinguish between fundamental procedural breaches (which do undermine the prosecution) and immaterial errors (which might be treated as harmless). Nevertheless, when a mandatory procedure in the RTA or accompanying regulations is flouted, it can indeed be a complete defence. Each case turns on its facts: the defence will argue the breach prevents a safe conviction, and the court decides if the error justifies acquittal.

(d) Lack of Evidence / Unreliable Breathalyser Readings

The prosecution must prove beyond reasonable doubt that the defendant was driving over the limit. If the evidence is lacking or unreliable, the defendant must be acquitted. Sometimes this overlaps with procedural issues above, but it also covers situations like faulty equipment, analytical errors, or insufficient proof that the defendant was the driver.

Challenging the Breathalyzer or Lab results: Modern breathalyzer devices are generally reliable, but they are not infallible. A defence can be mounted by attacking the accuracy of the alcohol reading. The defence might claim the machine was malfunctioning, not calibrated properly, or operated incorrectly, leading to a false high reading​. For instance, if maintenance records show the device was overdue for service or had a history of faults, this could cast doubt on the result. In cases involving blood or urine, chain-of-custody errors or laboratory mistakes (mixing up samples, using incorrect preservatives, etc.) can occur​. The high-profile Randox Testing Services scandal is an example where hundreds of blood test results were found unreliable due to laboratory tampering, leading to cases being dropped​. While that is an extreme scenario, it underscores that evidential samples can be challenged.

English case law confirms the right of a defendant to dispute the reading. In Cracknell v Willis [1988] 1 AC 450, the House of Lords held that a defendant may challenge the breathalyzer result by giving evidence of their own alcohol consumption and arguing the reading must be wrong​. In that case, it was recognized that if a driver’s recollection of only a small amount of alcohol (insufficient to be over the limit) is credible, it calls into question the machine’s accuracy​.

The court must then assess whether it is sure the machine was correct. Similarly, the CPS guidance notes that if the defence presents any cogent evidence that the breathalyzer reading might be erroneous, the court should only convict if satisfied the device “provided a reading upon which they can rely”​. In other words, a reading is not gospel; it can be impeached by evidence to the contrary. However, simply saying “I don’t believe I was that drunk” without evidence will carry little weight – typically an expert witness is needed to identify a specific issue with the device or process.

No Evidence of Driving / Identity: Another angle of “lack of evidence” is if the prosecution cannot prove the defendant was actually driving (or in charge) at the relevant time. For example, if a car is found crashed and the owner is intoxicated nearby, the Crown must link the driving to that individual. If there is reasonable doubt about who was driving (perhaps someone else was driving and fled, or multiple people were present), then the charge fails. This is a factual defence: simply put, the defendant says “I wasn’t the driver”. The burden remains on the prosecution to prove that he was. In a scenario where no officer witnessed the driving, the prosecution might rely on circumstantial evidence (ownership, admissions, etc.). The defence can exploit any gaps – for instance, by pointing out the lack of fingerprints, keys in someone else’s possession, or providing an innocent explanation for why the defendant was found intoxicated at the scene (but not driving). If the magistrates or jury are not sure the defendant drove, they must acquit. (This defence is less about a legal rule and more about holding the Crown to strict proof of each element of the offence.)

Private Property: A related defence is that the vehicle was not driven on a road or public place – which is a required element of the offence. If a vehicle was driven only on genuinely private land (with no public access), then it’s not within the scope of RTA 1988 s.5​. For instance, if someone only drove within a farmer’s field or a private driveway while intoxicated, that might not be an offence (though caution is needed: many places that feel private may legally count as public places if the public can access them). Raising this defence means arguing the location was not “public.” The burden is on the prosecution to prove the driving took place on a road/public place, so if the defence shows it was on private property, the court may rule the element unproven and dismiss the case.

Raising these issues: Challenges to evidence reliability are often raised at trial through cross-examination of the prosecution witnesses (e.g. asking the officer about the device’s calibration checks) and by calling defence experts. If successful, the court may find that the prosecution has not proven the case beyond reasonable doubt. A dramatic example occurred in some jurisdictions where large numbers of cases were thrown out due to systemic breathalyzer problems (though such large-scale failures have mostly occurred abroad, like several U.S. states, rather than in England). Nonetheless, English defendants have on occasion secured acquittals by uncovering machine faults or procedural lapses. Courts will ultimately weigh the credibility of the machine’s reading against any contrary evidence. As noted, if a defendant’s recollected consumption is inconsistent with the reading, and especially if an expert supports the possibility of an error, the court may have a reasonable doubt. The presumption is that devices function correctly (there is even a common law presumption that a mechanical device was working properly unless evidence shows otherwise​), so the defence often needs to produce that contrary evidence. When they do (for instance, showing the device failed a calibration check), it can tip the balance in the defendant’s favor.

(e) Medical Conditions Affecting Breath Test Results

Certain medical conditions or physiological factors can affect breath test results or even the fact of being “in control” of one’s actions, providing potential defences or at least reasonable doubt in drink driving cases. These defences often require expert medical testimony to explain the condition and its effects:

• Diabetes (Ketoacidosis): A diabetic person, especially with uncontrolled diabetes, can experience ketoacidosiswhich produces acetone in the breath. Breathalyzer devices cannot always distinguish acetone from ethanol – some older models or certain conditions might register acetone as alcohol. Studies and cases have noted that diabetic ketoacidosis can lead to a false-positive reading on breath tests​. For example, a diabetic in hypoglycemia might exhibit symptoms similar to intoxication (dizziness, slurred speech) and also have “fruity” acetone breath which a device reads as alcohol​. If evidence shows the defendant’s high reading could have been partly or wholly due to a medical condition (rather than alcoholic drinks), this can inject reasonable doubt. A medical expert could testify that the defendant’s blood sugar was extremely high and that acetone levels were present which might confuse the machine. This line of defence challenges the reliability of the result due to the defendant’s physiology.

• GERD or Acid Reflux: Gastroesophageal reflux disease (GERD) or vomiting can bring alcohol from the stomach into the mouth/throat after drinking. Even if the person drank only a safe amount, residual mouth alcohol from recent regurgitation can cause an inflated breathalyzer reading. Proper police procedure tries to mitigate this (by observing a waiting period as noted above). But if a defendant has a condition that causes frequent reflux, or if they happened to burp/vomit prior to the test, the accuracy is compromised. A defence could argue that the reading reflected mouth alcohol rather than deep lung air. Medical evidence or even the officer’s notes (e.g. noting the suspect burped or tasted acid) could support this defence.

• Asthma or Lung Conditions: These don’t create false alcohol, but they might impede giving a proper sample. If a person has severe asthma or another condition that prevents a long exhalation, they might not physically be able to provide the required volume for the breath machine. This could be a “reasonable excuse” for failing to provide a specimen (a defence to a charge of failing to provide under RTA s.7(6)). In the context of providing a sample but getting a reading, this is less of a defence and more an explanation if they are charged with refusal.

• Auto-brewery Syndrome: In extremely rare cases, individuals have claimed to have “auto-brewery” syndrome – a condition where the gut ferments carbs into alcohol, causing endogenous intoxication. While documented in medical literature, it’s not widely accepted in courts as a defence unless very clearly proven. Theoretically, if someone had this condition, they might be over the limit without having knowingly consumed alcohol. This would border on a lack of mens rea or even insanity/automatism defence (since the act of consuming alcohol wasn’t voluntary). However, this defence is exceptionally uncommon and would require strong medical proof.

• Automatism / Unconsciousness: Although not exactly about breath test results, a medical episode that causes a person to drive in an unconscious or involuntary state could be a defence. For instance, a diabetic might suffer a hypoglycemic blackout (low blood sugar can cause confusion or even “automatic” behavior). If a person in such a state drove without conscious control, they could argue non-insane automatism – that the act of driving was not under the control of their mind. This is also rare in practice for drink cases, because typically the presence of alcohol complicates the picture (and if they knowingly drank, a court might say the ensuing automatism was self-induced). But conceptually, if someone were, say, sleep-driving (like sleepwalking but got in a car), that could be a defence of automatism​. Again, this is exceedingly uncommon, but it has been mentioned in legal commentary as a possible defence (e.g. automatism was noted as a theoretical defence in one solicitor’s guidance, such as a case of sleep-walking or a diabetic episode causing unconscious driving)​

In summary, medical defences either challenge the reliability of the alcohol reading (by suggesting a physiological cause for a false positive) or they negate the voluntary nature of the offence. To use these defences, the defence would introduce medical evidence at trial. If successful, they either create a reasonable doubt about the accuracy of the readings (leading to not guilty), or in the case of automatism, they negate the actus reus (also leading to acquittal). It’s important to differentiate these from “insanity” – a long-term mental disorder would rarely be relevant here, whereas transient medical conditions as described are more likely arguments. While uncommon, these medical angles are a reminder that not every high breathalyzer reading is caused by alcoholic beverages, and courts must consider such evidence when presented.

(f) Other Defences and Special Situations

Beyond the major categories above, a few other defences or quasi-defences should be noted:

• Mistake or Spiked Drinks: If a person genuinely did not realize they were consuming alcohol (for example, someone spiked their drink without their knowledge), they might lack the necessary intent or mens rea. However, drink driving is largely a strict liability offence, so not intending to consume alcohol is generally not a full defence – the offence doesn’t require intent to drive drunk, just the act of driving over the limit. Instead, a spiked drinks scenario is usually raised as a special reason to avoid punishment (see below). The court can still convict but may decide not to impose the usual mandatory ban if it finds the alcohol was unknowingly ingested. So “my drink was laced” won’t typically get an acquittal, but it can spare the licence.

• Duress by Threats: In an unusual case, if a person was literally forced by another person to drink and drive (e.g. at gunpoint someone forces them to consume alcohol and drive), that could be a defence of direct duress (by threats of violence). This would be extremely rare and overlaps with duress of circumstances mentioned earlier. English law would likely treat it similarly to duress of circumstances – if a defendant can show they were compelled by threats of death or serious harm to commit the offence, it may exonerate them​.

• No Offence Actually Committed: It sounds obvious, but sometimes charges are laid when, in law, no offence took place. For example, operating the vehicle might not mean what laypeople assume – if someone was just sitting drunk in the driver’s seat without starting the engine, is that “attempting to drive” or “in charge”? It could be “in charge,” but the defence might be that they had no intent to drive (tying into the s.5(2) defence). Another example: if the breath reading was under the limit but the police charged the person under the impairment offence (Section 4), the defence could simply be that there’s no evidence of actual impairment. Clarity on the charge and the evidence can sometimes reveal that the prosecution cannot prove the specific offence beyond reasonable doubt.

• Technical Loopholes: Over the years, defence lawyers have identified various technical “loopholes” in the complex drink drive laws – for instance, arguments over whether the roadside procedure was lawful, or whether the timing of the breath tests adhered to statutory requirements. Many of these have been closed by legislative amendments or higher court rulings. As an example, prior to 2015, if a suspect’s breath reading was only marginally over the limit (between 40–50 µg), they had a statutory right to demand a blood or urine test instead (the so-called “statutory option”). That option was removed by law, so it’s no longer a defence to say “I should have been offered a blood test” if the reading was in that range – because the law changed. Thus, some defences that once existed are not available today. It’s important for lawyers to be up to date on such changes.

• Special Reasons (not a Defence but relevant): Though not a defence (since one is still guilty), if convicted a driver can argue there are “special reasons” not to impose the usual penalty. Special reasons are circumstances connected with the offence that do not excuse it in law but mitigate it. Common examples in drink driving include: driving in an emergency, shortness of distance driven, or spiked drinks​. If successful, a special reasons finding can prevent a mandatory disqualification even though the person is convicted. While not a route to acquittal, it’s worth mentioning because scenarios like driving under duress of circumstances or in a medical emergency often end up being presented as special reasons if they fall short of a full defence. The court then retains the conviction but can show leniency in sentencing.

In summary, the defences to a drink driving allegation in England and Wales cover a broad spectrum – from strict legal arguments about procedure and evidential presumptions to human stories of emergencies and mishaps. The availability of each defence depends on the facts of the case, and defendants will usually need to substantiate any such defence with evidence (their own testimony, witnesses, experts, documents, etc.). Next, we consider when and how in the legal process these defences are raised.

3. The Legal Process and Stages for Raising Defences

A drink driving case follows the usual criminal procedure, with some special features due to the technical evidence involved. We can break the process down into stages and consider where each type of defence might be deployed:

a. Investigation and Charge: The process typically begins at the roadside (a breath test by police if they suspect drinking) and continues at the police station with evidential breath, blood, or urine tests. At this investigatory stage, a suspect might already lay the groundwork for certain defences. For instance, if they have a post-driving consumptiondefence, they should mention to the officers as early as possible that they drank after driving (so that it’s recorded contemporaneously, lending credibility). Similarly, if there was an emergency reason for driving, telling the police that reason at the time can be important. The police will collect evidence (witness statements, breathalyzer printouts, CCTV, etc.). If procedural missteps occur here (like failing to follow proper testing protocol), the defence will note these for later.

After the evidential test, if the reading is over the limit (or if refused), the suspect is usually charged. At the moment of charge or soon after, the CPS will review evidence. In some cases, the defence can make representations to the CPS early on – for example, providing evidence of a technical defence (like a calibration issue) that might persuade the prosecution to drop the case before trial.

b. Pre-Trial (First Appearance and Case Management): Drink driving offences are dealt with in the Magistrates’ Court. The first hearing is for the defendant to enter a plea. If they plead Not Guilty, this triggers the case management process. At this stage, the defence should identify the issues in dispute. Courts encourage early identification of defences. For example, if the defence is a hip flask defence, the defence team will usually notify the prosecution that they will be contesting the s.15 presumption and possibly provide notice of any expert evidence they intend to use (expert reports often have to be disclosed in advance). Similarly, if the defence plans to argue duress of circumstances, it should be flagged, because it may affect what evidence is needed (the prosecution might inquire into the alleged circumstances).

During pre-trial management, deadlines are set for disclosure. The prosecution must supply all the evidence (statements, machine printouts, calibration certificates, etc.) to the defence. If the CPS fails to meet these deadlines – a common issue – the defence can later ask the court to exclude evidence or even dismiss the case​. For instance, if the CPS hasn’t served the lab report in time, the defence will highlight that. Pre-trial, the defence may also make specific requests for disclosure – for example, maintenance records of the breathalyzer, CCTV from the station, bodycam footage (which could show whether procedure was followed or what the defendant said at the scene). The CPS might resist some of these (and, as the CPS guidance notes, courts require a “proper evidential basis” for broad disclosure requests of machine records​). Any disputes on disclosure could be handled in pre-trial hearings.

If there are purely legal arguments to be made (for example, a claim that the charge is invalid, or an abuse of process because of some failing), the defence might raise them via a preliminary motion. However, typically defences like those discussed are fact-driven and will be dealt with at trial, not thrown out beforehand (unless the prosecution concedes something).

c. Trial (Magistrates’ Court): At the trial, the prosecution presents its evidence first. This usually includes the officer who conducted the breath test (to produce the machine printout and certify the procedure) and any other witnesses (perhaps the arresting officer who noted driving behaviour, etc.). It may also include the analyst (for blood cases) or just the documentary certificate if unchallenged. The defence then gets to cross-examine these witnesses. This is where procedural defences and reliability challenges are often pursued: the defence lawyer will question the officer in detail about each step – “Did you give the statutory warning? Did you observe the 20-minute period? Are you sure the machine was functioning correctly? Did you offer my client a portion of the blood specimen?” – trying to uncover any deviation or weakness. If the officer admits a mistake (or is unsure), the defence may later argue that this creates reasonable doubt or warrants exclusion of evidence.

After the prosecution case, the defence can make a submission of “no case to answer” if they believe the evidence is insufficient in law (for example, if no proper evidence of the blood-alcohol level was adduced because a certificate was ruled inadmissible). If the magistrates agree, the case is dismissed at that point without the defence even needing to present evidence.

If the case proceeds, the defence presents its evidence. This is when defences like hip flask or duress or medical conditions come to the fore. The defendant will usually testify on their own behalf, explaining the relevant facts (e.g. “I crashed and then drank whisky from my flask out of shock” or “I only drove because of the emergency”). Any expert witnesses for the defence (toxicologists, medical experts) will also give evidence at this stage, supporting the defence’s theory (a toxicologist might testify that, based on calculations, the defendant’s blood alcohol at the time of driving would have been under the limit​; a doctor might testify about the defendant’s medical condition affecting the reading). The prosecution can cross-examine these defence witnesses, attempting to undermine their credibility or conclusions.

After all evidence, both sides make closing submissions, arguing how the law applies to the facts. The defence will remind the court of any prosecution failures (e.g. “the officer never gave the mandatory warning, so you cannot rely on the breath test result”) and argue that their defence has been made out or at least raises a reasonable doubt. In magistrates’ court, the magistrates (or district judge) then retire to consider a verdict.

It’s worth noting that in drink driving trials, magistrates often have to consider conflicting expert evidence (prosecution vs defence toxicologists) or weigh a defendant’s testimony against the inference from the breath machine. They also have to apply any legal burdens correctly (for example, remembering that for the hip flask defence, the defendant bears the burden on balance of probabilities to establish the post-driving drinking story​). If a defence like duress of circumstances is raised, the magistrates must decide if the evidence establishes that defence (there’s no jury here, so magistrates both find facts and apply the law). Magistrates’ legal advisers can assist with the proper legal tests (e.g. what constitutes duress, what the statutory defences require, etc.).

d. Outcome and Appeal: If the defendant is acquitted, the matter ends (the prosecution generally cannot appeal an acquittal in a magistrates’ trial, except on a point of law by case stated). If the defendant is convicted, the case moves to sentencing (where, as mentioned, they might argue special reasons to mitigate the penalty). However, the defendant has a right to appeal the conviction.

Appeals can be of two types:

(1) Appeal to the Crown Court (which is essentially a re-hearing of the case if it was a summary conviction). The defence can re-argue their case in front of a Crown Court judge and magistrates, present their defences anew, and the Crown Court can reach its own conclusion, potentially overturning the conviction.

(2) Appeal by way of case stated to the High Court (Administrative Court) on a point of law or jurisdiction. For instance, if the magistrates misinterpreted the law (say, they wrongly ruled that duress of circumstances is not available for a drink driving charge, or they placed the burden of proof on the defendant incorrectly for a certain defence), the defence can appeal to the High Court to correct that error. An example is R v Martin (1989) – although that was a Crown Court trial on a different charge, the principle is similar: the appeal court said the trial court erred in disallowing a duress defence​. Similarly, if magistrates refused to consider a hip flask defence or got the standard of proof wrong for it, an appeal court could overturn the conviction.

Additionally, the prosecution or defence can appeal specific rulings (like evidentiary rulings) via case stated or judicial review if something clearly went awry, though in magistrates’ court trials this is less common during the trial – usually such issues are bundled into a post-trial appeal.

e. Examples of Defences in the Process: To illustrate when defences arise:

• A hip flask defence typically is presented at trial with evidence, but preparation happens pre-trial (obtaining expert reports, notifying prosecution). If the prosecution is made aware early and the defence evidence is strong, sometimes the prosecution may even reconsider proceeding if they believe the defence will succeed (though often they will let it go to trial and let the court decide).

• A procedural breach defence might be argued through cross-examination and possibly a legal submission to exclude evidence. For example, if during trial it’s revealed that the suspect was not given a blood specimen portion to keep, the defence can submit that the blood test evidence should be excluded for non-compliance with the statutory procedure. If the court agrees, the prosecution may be left with no admissible evidence of the alcohol level and thus “no case.” Such arguments could also be raised in a pre-trial motion to exclude (under PACE s.78) if the facts are clear enough from the outset (some defence lawyers will file an application to exclude evidence before trial if, say, the police station CCTV clearly shows no warning was given or similar).

• A duress/necessity defence will be decided at trial based on the evidence. There is usually no pre-trial way to determine it (unless the prosecution agreed the facts, which is unlikely). The court will hear the defendant’s account of the threatened harm and decide.

• Medical condition defences similarly come out through evidence at trial (though sometimes, if the prosecution is convinced by medical reports provided beforehand – for instance, proof that a diabetic defendant was in ketoacidosis – they might drop or reduce the charge).

During the trial, both prosecution and defence may refer to case law to support their positions on the law. For instance, the defence might cite DPP v McKeown to argue that a technical breach should invalidate the test, or the prosecution might cite it to say “minor technical errors that don’t affect the result shouldn’t acquit”​. Or in arguing duress, cases like Martin would be referenced as precedent that the defence is available. Magistrates’ legal advisors or judges will consider these legal precedents in their rulings.

f. Post-Acquittal Issues: If a defendant is acquitted using any of these defences, that’s the end of the criminal matter. Notably, an acquittal on a technical ground (say, evidence excluded) is still a full acquittal. The prosecution cannot re-charge the offence. However, if it was a procedural acquittal (like evidence thrown out), the prosecution might examine what went wrong to avoid it in future cases (e.g. police retraining on procedure). In rare instances, if the acquittal was due to what the prosecution perceives as an erroneous magistrates’ decision on law, they could try a case stated appeal to the High Court to clarify the law – but they cannot overturn the acquittal of that individual due to double jeopardy protections.

In summary, defending a drink driving charge is a multistage effort: identifying the right defence early, gathering supporting evidence (often with experts), making necessary disclosure requests, and then executing the defence through cross-examination and evidence at trial. If unsuccessful at first instance, the fight can continue on appeal. The process stage influences how the defence is presented: some are argued as legal motions, others as factual persuasion before the trier of fact.

4. Case Law Illustrations

To understand how these defences operate in practice, it’s helpful to look at relevant case law where defendants raised these defences and how the courts responded. Below are several notable cases in England and Wales that illustrate the application of the above defences:

• R v Kitson [1955] 39 Cr App R 66: This early case famously demonstrates the (former) strict approach to necessity. Mr. Kitson was a passenger who woke up alone in a moving car rolling down a hill; he was intoxicated and steered the car to a stop to prevent an accident. Despite what seems like a heroic act, he was convicted of drunk driving. The court did not accept necessity as a defence, essentially taking a zero-tolerance stance (at that time) that no matter the reason, he was “driving” while intoxicated​. Kitson’s case is often cited to show that historically the courts refused a necessity defence for driving offences. (Modern law would likely view these facts more sympathetically under duress of circumstances, as discussed next.)

   

• R v Martin [1989] 1 All ER 652 (88 Cr App R 343): A leading case on duress of circumstances, Martin signaled a shift in the law. Martin, who was disqualified from driving at the time, drove his stepson to work while over the limit because his wife threatened to commit suicide if he didn’t (she was in a distressed, suicidal state). The trial judge ruled Martin could not use necessity/duress as a defence, but on appeal the Court of Appeal disagreed and quashed the conviction​. The court held that duress of circumstances should have been left to the jury – if they believed Martin’s wife genuinely threatened suicide and a sober person might have felt compelled to drive in that situation, then a defence was available. Martin established that such a defence exists for driving cases, effectively carving out an exception to the rigidity exemplified by Kitson. It also laid down a test (later refined in other cases) for duress of circumstances: an objective test of whether a sober person of reasonable firmness, sharing the defendant’s characteristics, would have done the same. While Martin’s case involved driving while disqualified, the principle has since been applied to drink driving scenarios involving emergencies or threats.

   

• Parker v DPP [2000] (reported [2001] RTR 240): This case involved a “hip flask” defence and is notable for its human rights dimension. Mr. Parker had been involved in a car accident and consumed alcohol afterwards (the classic hip flask scenario). He was charged under RTA 1988 s.5 and sought to rely on post-accident drinking to rebut the presumption of his alcohol level at the time of driving. The divisional court (High Court) considered whether the statutory presumption of RTOA 1988 s.15(2), and the placing of a legal burden on Parker to prove his post-driving consumption, violated the presumption of innocence under Article 6(2) ECHR. The court ultimately held the presumption and reverse burden to be lawful and compatible with Article 6​. Citing the European case Salabiaku v France, it reasoned that evidential presumptions are permissible if kept within reasonable limits, and given the importance of road safety, this one was justified​. Parker’s own conviction turned on whether the magistrates believed his hip flask story and evidence. The case underscores that while a defendant can have a full defence by proving post-driving drinking, the law does require them to meet that burden, and that requirement has been upheld by the courts. Key point: Parker confirms that a ‘hip flask’ defence is tightly constrained – the defendant must prove it, and merely asserting it without solid proof won’t succeed. It also affirms the legality of the statutory scheme surrounding this defence.

• Dawson v Lunn [1986] RTR 234: This is an earlier case on post-driving alcohol (under older legislation prior to the RTOA 1988, but similar in effect). Mr. Dawson claimed he drank after driving. The case is often cited for articulating what a defendant must show to rebut the presumption. It set the stage for how the later statutory defence in s.15 would work. The decision in Dawson emphasized that if credible evidence of post-incident drinking is presented, the court should not automatically assume the reading reflects the driving time. In practice, Dawson v Lunn is frequently mentioned alongside the statutory provisions as an example where a defendant successfully cast doubt on the alcohol level at the time of driving.

• Cracknell v Willis [1988] 1 AC 450: A House of Lords case that is a cornerstone for challenging the reliability of readings. In Cracknell, the defendant argued that given the small amount of alcohol he had consumed, the high breathalyzer result must be wrong. The House of Lords held that a defendant is entitled to challenge the prosecution’s evidence of blood alcohol by giving evidence of the amount consumed and any other relevant facts​. Lord Ackner in that case observed that if a defendant’s account of their drinking (e.g., “I only had one pint”) is believed and indicates they ought to be under the limit, then unless the prosecution disproves that or undermines it, the court could not be sure of guilt. Essentially, Cracknell v Willis put to rest any notion that machine evidence is incontrovertible. It confirmed that courts must evaluate the totality of evidence – both the machine reading and the defendant’s evidence – and if after doing so there’s doubt about the accuracy of the machine’s figure at the time of driving, the benefit of that doubt goes to the defendant. This case is often cited by defence lawyers when arguing that the machine might be wrong. (It’s also referenced in CPS guidance, which notes that courts must be satisfied the device is reliable, citing Cracknell​.)

   

• DPP v McKeown; DPP v Jones [1997] AC 310: A joint appeal in the House of Lords concerning technical procedural compliance. In one of these cases, the intoxilyzer machine’s internal clock was running slow, so the printout showed an incorrect time (over an hour off). In the other case, a suspect had produced one valid breath sample over four times the limit, then refused to provide the second. The question was whether these technical issues invalidated the convictions. The Divisional Court had initially quashed the convictions, but the House of Lords reinstated them. Lord Hoffmann reasoned that the inaccuracy of the clock did not prejudice the defendant– everyone knew the real time and it had no bearing on the alcohol reading​. As for the single breath sample, since the suspect deliberately refused the second, the evidence of the first (being so clearly over the limit) was admissible. DPP v McKeown is often cited by prosecutors to counter over-technical defences: it stands for the idea that not every deviation is fatal – only those that cause injustice or breach statutory safeguards. It illustrates the courts’ balancing act: insisting on important procedural protections, but not allowing irrelevant technicalities to overturn convictions. After McKeown, Parliament also adjusted some regulations to clarify procedures (and today, failing to provide the second specimen is itself an offence, so either way the offender is culpable).

   

• Sheldrake v DPP [2004] UKHL 43, [2005] 1 AC 264: This case dealt with the human rights compatibility of reverse burdens in road traffic offences. Mr. Sheldrake was charged with being “in charge” of a vehicle while over the limit (RTA 1988 s.5(1)(b)), and the statutory defence in s.5(2) (no likelihood of driving) places a burden on the defendant. Like Parker, Sheldrake argued this was incompatible with Article 6(2). The House of Lords (Lord Bingham) held that the burden could be read down as merely requiring the defendant to raise an issue (an evidential burden) rather than a full legal burden, to be compatible – but even if a legal burden, it was proportionate. Sheldrake’s conviction was restored on the facts because he couldn’t show no likelihood of driving. This case is a bit technical, but it confirms that even for the in charge defence, the defendant carries the risk of non-persuasion. The Law Lords justified this on the basis that the offence is not too stigmatic and the maximum penalty is moderate, so a reverse burden is not a gross breach of fair trial rights​ (similar reasoning as in Parker). For our purposes, Sheldrake underscores that statutory defences like s.5(2) and the hip flask provision do expect the defendant to prove something, and the courts generally uphold that scheme.

   

• Rose v DPP [2022] EWHC (Admin) and DPP v Manchester & Salford Magistrates’ Court [2017] EWHC 3719 (Admin): These are more recent High Court decisions dealing with disclosure and breathalyzer source code issues. Without going into detail, they show the court’s attitude towards so-called “fishing” defences – where the defence requests extensive technical data about the breath machine hoping to find an error. The courts have mostly been unsympathetic unless the defence can point to a specific reason to suspect a malfunction​​. In Rose, the defendant wanted the raw data from the breath machine memory; the court held that s.15(2) presumption didn’t oblige the prosecution to produce that, and as long as the approved device printout is there, that suffices​. These cases illustrate that while defendants can challenge evidence, they need some foundation – sweeping requests or speculative arguments about machine infallibility generally won’t succeed without evidence of a particular issue.

Each of these cases sheds light on how drink driving defences operate: Kitson and Martin show the development of emergency/duress defences; Parker, Dawson, and Cracknell illuminate the handling of the hip flask and reliability challenges; McKeown shows the limits of technical defences; Sheldrake addresses the burden of proof in statutory defences. They collectively demonstrate that while the law surrounding drink driving is strict, it is also nuanced – allowing genuine defences where appropriate but requiring defendants to meet evidential standards for those defences.

Conclusion: In England and Wales, the legislative framework (principally the Road Traffic Act 1988 and Road Traffic Offenders Act 1988) creates a robust mechanism to prosecute drink driving, aided by presumptions and strict procedures. However, it also carves out specific defences and the common law overlays additional safeguards. Defendants can avoid conviction if they can successfully invoke a defence such as post-driving alcohol consumption, necessity/duress, or show that the evidence against them is flawed. The legal process provides multiple opportunities to raise these issues – from the investigative stage through to appeals – ensuring that convictions are secure and just. The case law illustrates that courts are willing to acquit where a valid defence is proven (or where doubt is raised), but also that they scrutinize such claims carefully. Ultimately, each case will turn on its facts, but the defences discussed above form the key arsenal for any lawyer defending an allegation of drink driving in England and Wales.

Sources:

• Road Traffic Act 1988, Sections 4, 5 & 7 (statutory offences and procedures).

• Road Traffic Offenders Act 1988, Sections 15 & 16 (evidential provisions on specimens and presumptions)​.

• CPS Legal Guidance – Road Traffic: Drink and Drug Driving (2020) – outlines offence elements and addresses defences and challenges​.

• Case Law:

  • R v Kitson [1955] 39 Cr App R 66 – necessity not accepted for drunk driving​.
  • R v Martin [1989] 88 Cr App R 343 – duress of circumstances recognized (wife’s suicide threat)​.
  • R v Conway [1988] QB tot – (not quoted above, but a CA case allowing duress of circumstances for reckless driving).
  • Parker v DPP [2001] RTR 240 – hip flask reverse burden held compatible with ECHR​.
  • Dawson v Lunn [1986] RTR 234 – evidential burden on defendant to prove post-driving drinking (pre-RTOA but relevant)​.
  • Cracknell v Willis [1988] 1 AC 450 – HL: defendants may challenge breath readings; courts must be satisfied of reliability​.
  • DPP v McKeown; DPP v Jones [1997] AC 310 – HL: minor procedural errors (e.g. faulty clock) did not invalidate conviction​.
  • Sheldrake v DPP [2005] 1 AC 264 – HL: “drunk in charge” defence’s reverse burden read as proportionate (Article 6 compliant).
  • Rose v DPP [2022] EWHC – administrative court on breathalyzer data disclosure (not in detail above).
  • Brown v Stott [2003] 1 AC 681 (PC) – affirmed use of compelled roadside statements in drink driving (related to self-incrimination, not discussed above but part of context).

• Additional reference: Law Commission Report on Offences Relating to Motor Vehicles (for historical context on necessity and drink driving).