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Court rule that fuelled Post Office’s prosecution rampage: Everything you need to know

This guide contains essential information about the legal rule that enabled the Post Office to wrongly prosecute subpostmasters

Karl Flinders
By
Published: 16 Dec 2025

In 1999, a new rule on the use of computer evidence in court was introduced, which unintentionally went on to enable the Post Office to prosecute and destroy the lives of innocent subpostmasters.

It was a few years earlier, in 1995, when the Law Commission put out a call for evidence on a proposed change to Section 69 of the 1984 Police and Criminal Evidence Act, which stated that computer-based evidence should be subject to proof that the computer system was operating properly at the time of the alleged offence.

The Law Commission wanted feedback on proposals to change this and introduce a presumption that a computer system has operated correctly, unless there is explicit evidence to the contrary.

The 1995 consultation sought the views of prosecuting organisations on the proposed change to the rules on the use of computer evidence in court. One of these organisations was the Post Office. In its response, the Post Office’s criminal law division said the rule at the time was “somewhat onerous from a prosecution viewpoint” and that it supported the change.

A Post Office legal representative wrote: “I consider that computer evidence is, in principle, no different from any other sort of evidence, and it should, in general terms, be admissible, so that any argument in court would be related to its weight rather than its admissibility.

“I therefore consider that there should be a presumption that the machine is in working order, etc, and if the defence wish to argue otherwise, then clearly, they should be able to do so. At present, I therefore consider the evidential requirements to be far too strict and can hamper prosecutions.”

Read more: Post Office supported 1999 law change that eased prosecutions using computer evidence.

Legal rule as a business tool

The rule was put into effect in 1999, which coincided with the introduction of the Horizon system computerising accounting for the entire Post Office branch network.

Not long after that, the Post Office went on a prosecution rampage. It already had private prosecution powers and didn’t require the police or the Crown Prosecution Service. Now it didn’t need robust evidence, only the evidence from its faulty computer system.

In the seven years between 1991 and the year before Horizon’s introduction, an average of six subpostmasters or staff were convicted per year. This was a 766% increase in the prosecution rate.

The Post Office threatened prosecution to bully subpostmasters into covering unexplained shortfalls that didn’t actually exist. This was made easy because the Post Office didn’t have to prove Horizon was working at the time of the shortfall.

Read more: IT scandal exposes legal rule that made it easy for Post Office to prosecute the innocent.

High Court raises questions

The controversial rule, along with the Post Office’s private prosecution powers, became major talking points after a High Court battle between subpostmasters and the Post Office ended in December 2019. This group litigation order (GLO) proved that accounting shortfalls, for which subpostmasters were blamed, were actually caused by computer errors.

Soon after, in June 2020, convictions of subpostmasters based on evidence from the Horizon system were sent to the Court of Appeal. Meanwhile, the government was asked to look at the rules around private prosecutions. 

Read more: Criminal Cases Review Commission sends 47 more subpostmaster cases to Court of Appeal and asks government to review private prosecution powers.

Government agrees review

The Post Office scandal has created legal landmarks, including the government’s decision to overturn the convictions of hundreds of people who were prosecuted based on data from Horizon. Less dramatic, but of huge significance, private prosecutions have been pulled back.

Read more: Government agrees to change private prosecution rules that were abused by the Post Office in its pursuit of subpostmasters wrongly accused of financial crimes.

Rules on evidence are still the same, however. In 2023, following a large number of subpostmaster convictions being overturned, a parliamentary campaign to change the rules on computer evidence began to build a head of steam.

Read more: MP calls for review of computer evidence rule which led to subpostmasters being wrongly convicted.

Public pressure

Everything sped up in January 2024, following the broadcast of ITV’s drama about the Post Office scandal. Nothing new was revealed, but it brought the harrowing stories of subpostmasters to the general population, rather than campaigners and a few journalists.

Everybody knows the profound impact the drama had, with the government acting quickly to overturn subpostmaster convictions, but it also stirred more urgency about addressing the rules on computer evidence, which had enabled the Post Office to prosecute subpostmasters without robust evidence, or with unreliable evidence.

Read more: Change to rules on computer evidence will be an ‘outcome’ of Post Office scandal.

Read more: Lords debate amendment to law on use of computer evidence in light of Post Office scandal.

Call for evidence

It was January 2025 when the government called on experts to guide it on what needs to change – a move welcomed by the people that suffered at the hands of the current rules. 

Read more: Government calls for expert views on computer evidence to learn lesson from Post Office scandal.

Review of legal rule on computer evidence long overdue, say Post Office scandal victims.

Computer and law experts respond to call for opinions on computer evidence.

What will emerge?

It is not straightforward, according to court of appeal judge Peter Fraser, who took charge of the 2018/19 High Court battle between the Post Office and subpostmasters: “It’s very difficult to know now how courts in the future, the next 10, 20, 30 years, are going to deal with computer-generated evidence, or evidence from computers, or evidence about the operation of computers. What is important is being aware of what can be changed.

“There needs to be some flexibility in how we approach it because the current complexity is just going to get more and more marked,” he said, in an Inner Temple talk, The use of electronic evidence in the law.

Read more: No simple replacement for digital evidence rules, says Post Office Horizon trial judge.

What legal and IT experts have to say

Alistair Kelman, British barrister (retired) and technologist: How to solve the computer evidence problem.

James Christie, independent consultant and IT expert: Law Commission misrepresented experts when it changed rule on computer evidence.

Stephen Mason, retired barrister and expert on digital evidence: The cause of the Post Office Horizon scandal? The Law Commission? Judges? Lawyers?

Paul Marshall, barrister who represented former subpostmasters: Naivety of computer evidence leaves door ajar for more miscarriages of justice.

Stephen Castell, computer software and systems expert witness: A trial relying on computer evidence should start with a trial of the computer evidence.

Read more about the Post Office Horizon scandal

Read more on IT legislation and regulation