Charged With Drink Driving? Why a Breathalyser Reading Isn't Always the End of the Case

By MAJ Law on 06 October 2026

If you have been arrested or charged with drink driving, there is a good chance you have already been given plenty of advice. Friends may tell you to plead guilty. Someone at work might know somebody who “got off” a similar charge. Google will give you thousands of results. And perhaps the most common assumption of all is this:

“If the breathalyser produced a reading over the limit, there’s nothing you can do.”

That isn't necessarily true.

A breathalyser reading can be extremely important evidence, but it does not exist in isolation. A drink-driving prosecution can involve a chain of evidence and procedure, from the initial stop and roadside test through to the evidential procedure at the police station and the presentation of the case at court. Every case turns on its own facts. But where a defendant pleads not guilty, those procedures and the evidence supporting them can come under close scrutiny.

As Conor Johnstone, Director at MAJ Law, explains:

“Everyone assumes that because the machine printed a number, the case is over. It isn't. A number on a printout is only the start of the prosecution case.”

So what actually gets examined when a drink-driving case is defended? And how can something seemingly as simple as a breathalyser reading become a much more complicated legal case?

Read our blog below for more information on drink-driving offences. Alternatively, watch the full video here. 

Drink-driving cases are more technical than they appear

Most people encounter the end result: two breath readings and a charge. A specialist motoring solicitor sees everything that happened before, during and after those numbers were produced.

The prosecution must prove the offence to the required criminal standard and rely on admissible evidence. That can mean examining whether the statutory procedure was followed correctly, whether the evidential material supports the prosecution case and whether there are any legitimate issues surrounding the evidence.

Drink-driving cases can therefore involve questions concerning:

  • the roadside procedure;

  • the evidential procedure at the police station;

  • the MGDD paperwork;

  • evidential breath-testing equipment;

  • the timing and sequence of events;

  • statements from police officers;

  • body-worn video or CCTV where relevant;

  • medical or physiological issues;

  • mouth alcohol or contamination issues where genuinely supported by the evidence;

  • disclosure of relevant material; and

  • inconsistencies between documents, statements and other evidence.

One error will not automatically cause a prosecution to fail. Many procedural irregularities have no effect whatsoever on the ultimate admissibility or reliability of the evidence. But some issues can matter enormously. The key is knowing the difference.

The MGDD paperwork: one of the first places we look

One of the most important documents in many drink-driving investigations is the MGDD documentation. The Manual of Guidance on Drink and Drug Driving contains standard forms used by police forces in England and Wales when dealing with drink and drug-driving offences.

For an evidential breath procedure at a police station, MGDD Form A sets out the relevant procedure. To someone unfamiliar with drink-driving law, it can look like little more than police paperwork. To a specialist solicitor, it can provide a detailed record of what actually happened.

Conor explains:

“It's a document officers are completing during an active investigation, often late at night or in the early hours of the morning. We go through it carefully and compare it against the other evidence in the case.”

The defence may examine the paperwork for issues such as:

  • timings that appear inconsistent;

  • unanswered or incomplete sections;

  • differences between the form and an officer's statement;

  • whether relevant questions were asked and answers recorded;

  • the sequence in which procedures were carried out; and

  • whether the paperwork is consistent with other available evidence.

Crucially, a mistake on an MGDD form does not automatically invalidate a prosecution. But an apparent discrepancy may require an explanation and, depending on its nature and importance, could form part of a wider challenge to the prosecution evidence. That is why simply looking at the final breath reading tells only part of the story.

What about the evidential breath-testing machine?

The machine itself can also become an important part of the investigation. Evidential breath-testing instruments used by the police are subject to technical requirements and procedures. Where there is a legitimate issue concerning the instrument or the reliability of a particular result, the defence may investigate the relevant evidence and records.

This is very different from simply claiming: “The machine must have been wrong.”

A defence needs an evidential and legal basis. Depending on the circumstances of the individual case, issues surrounding the breath-testing procedure or instrument may require further investigation.

Conor explains:

“You don't challenge evidence simply for the sake of challenging it. You look at the evidence and ask whether the prosecution can properly establish the reliability of what it is relying upon.”

Where a genuine technical issue exists, specialist evidence may sometimes also be required.

Why the first court hearing can be so important

For somebody accused of drink driving, the first hearing may feel like little more than an administrative appointment. It isn't. If a not guilty plea is entered, the court needs to identify the real issues in dispute and manage the case towards trial.

CPS guidance specifically notes that following a not guilty plea to a summary drink-driving offence, the defence is expected to complete the Preparation for Effective Trial (PfET) process and clearly identify the real issues and areas of dispute. This makes early preparation extremely important.

A specialist solicitor should have considered the available evidence before deciding which issues genuinely need to be raised. In an appropriate case, that might include several separate matters rather than relying on one broad argument.

For example:

Issue 1: Is there a material problem within the MGDD procedure?

Issue 2: Is there a legitimate issue concerning the evidential breath-testing instrument or readings?

Issue 3: Is there evidence capable of supporting a mouth-alcohol or contamination issue?

Issue 4: Are there inconsistencies between the police paperwork, statements or other evidence?

Issue 5: Is further relevant material required from the prosecution?

Not every case will contain these issues, and raising weak arguments simply to create work is not good defence strategy. But where several genuine issues exist, identifying them early matters.

As Conor puts it:

“The first hearing sets the tone for what follows. The job isn't to throw five random arguments at the prosecution. It's to identify                       every legitimate issue in the evidence and make sure each one is properly dealt with.”

Disclosure: what does the prosecution have to provide?

Disclosure is another important, and often misunderstood part of criminal proceedings. Under the Criminal Procedure and Investigations Act 1996, the prosecution has duties concerning material which may reasonably be considered capable of undermining the prosecution case or assisting the defence case. Those duties continue as proceedings progress.

There is an important distinction here between the initial details of the prosecution case, the evidence upon which the prosecution intends to rely and unused material which engages the disclosure regime. In magistrates' court proceedings, there is not a universal statutory rule saying that the CPS has exactly 28 days from the first hearing to disclose every piece of evidence. Instead, prosecution disclosure must be dealt with as soon as reasonably practicable, while courts can impose specific case-management directions and deadlines for the service of material. Those deadlines matter.

Where relevant evidence or disclosure is missing, the defence can raise the issue with the prosecution and, where necessary, the court. The prosecution does not get an unlimited opportunity to prepare its case however it chooses. Criminal proceedings are governed by rules, statutory obligations and court directions designed to ensure cases are dealt with fairly and efficiently.

Can evidence be excluded if the police get something wrong?

Potentially, but again, it is not automatic.

Section 78 of the Police and Criminal Evidence Act 1984 (PACE) gives the court power to refuse to allow prosecution evidence where, considering all the circumstances, including how that evidence was obtained, admitting it would have such an adverse effect on the fairness of proceedings that it ought not be admitted. That is a significant legal power, but it should not be misunderstood.

A missed box, minor procedural error or late document does not automatically mean evidence will be excluded. The court looks at the circumstances and the effect upon fairness. Where a serious issue does exist, however, the admissibility of important evidence can become a central issue in the case. And if evidence crucial to proving an offence cannot ultimately be relied upon, that can have major consequences for the prosecution.

An overstretched criminal justice system

Drink-driving prosecutions also operate within a criminal justice system carrying a substantial caseload. Recent CPS operational data illustrates the scale.

During January to March 2026, there were 125,171 completed CPS prosecutions across all crime, while the overall live CPS caseload stood at 210,413 at the end of the quarter. The magistrates' court live caseload alone had risen to 118,151. Those figures don't mean a defendant wins simply because the system is busy. Far from it. But they demonstrate the sheer volume of cases being managed by prosecutors and the courts.

For defence solicitors, this makes careful preparation even more important. Documents must be checked, outstanding evidence identified and court directions monitored rather than simply assuming that everything required will automatically arrive.

“She told me it was her first month in court”

Conor recalls one recent drink-driving case which demonstrated just how quickly issues can develop at court.

“I walked into court to speak to the prosecutor and she was surrounded by defence solicitors, all waiting to discuss their cases with her.” The prosecutor, he says, explained that it was her first month appearing in court and she was dealing with a demanding list. “That's not a criticism of that prosecutor. It's the environment people are working in. But I raised several issues we had identified with the MGDD documentation in my client's case. Within a matter of minutes, the prosecution was no longer proceeding with it.”

It is an important distinction. The defence is not entitled to exploit invented technicalities, nor does a busy prosecutor mean a case will disappear.But a defendant is entitled to require the prosecution to prove its case properly.

This isn't about “loopholes”

The word “loophole” gets thrown around constantly in motoring law. It is often misleading.

  • Checking whether the police complied with the relevant procedure is not a loophole.
  • Examining whether evidence is reliable is not a loophole.
  • Requiring the prosecution to comply with disclosure obligations and court directions is not a loophole.
  • Challenging evidence where there is a proper legal basis to do so is not a loophole.

It is the criminal justice process.

Conor explains:

“We're not looking for tricks. We're holding the prosecution to the standard the law requires. They have to prove the offence properly. If there's a genuine problem with the evidence, the defendant is entitled to have that problem examined.”

The same principle applies whether somebody is accused of drink driving or a far more serious criminal offence. The burden of proving the prosecution case does not disappear simply because a machine has produced a number.

So, should you plead guilty if you're over the limit?

There is no responsible one-size-fits-all answer. If the evidence against you is sound and there is no viable defence, an early guilty plea can have important consequences for sentencing credit. Equally, somebody should not plead guilty to an offence they have not committed simply because they assume there is no point having the evidence professionally examined.

That distinction is crucial. A solicitor specialising in drink-driving law can review what happened, examine the available evidence and advise whether there is a genuine legal or evidential issue worth pursuing.

Sometimes there will be. Sometimes there won't.

But the decision should be based on the evidence -  not on something a friend said, an internet forum or the assumption that a breathalyser printout automatically answers every legal question in the case.

The key takeaway

If you have been charged with drink driving, don't panic,  but don't make assumptions either. A drink-driving prosecution can involve far more than the final number recorded at the police station.

The questions may include:

  • Was the correct procedure followed?

  • What does the MGDD documentation actually show?

  • Is the paperwork internally consistent?

  • Is there a legitimate issue concerning the evidential procedure or equipment?

  • Has relevant material been provided?

  • Have court directions been complied with?

  • Is there a genuine evidential or legal issue capable of affecting the prosecution case?

Those questions need to be answered by looking at the individual evidence.

As Conor says:

“Don't assume you're going to lose simply because you've seen a number on a printout. Get the evidence looked at properly and understand your position before deciding what to do.”

Charged with drink driving? Speak to MAJ Law

MAJ Law specialises exclusively in motoring law and regularly represents motorists facing drink-driving allegations.

If you have been arrested, charged or are due to attend court for a drink-driving offence, our team can review the circumstances of your case and explain your options.

We offer free initial advice with no obligation to instruct us.

Contact MAJ Law today to speak to a specialist motoring solicitor.

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