No simple replacement to digital evidence rules, says Post Office Horizon trial judge

The managing judge of the High Court battle that opened up the Post Office Horizon scandal has warned that while current rules on digital evidence need to change, they should not simply revert to their predecessor.

The Post Office’s use of flawed computer evidence to prosecute hundreds of subpostmasters who were unable to challenge it has put the spotlight on the current rules around the use of computer evidence in court, which has a presumption a computer has worked properly.

In 1999, the presumption was introduced into law on how courts should consider electronic evidence. The rule followed a Law Commission recommendation that courts should presume a computer system has operated correctly unless there is explicit evidence to the contrary. This replaced Section 69 of the PACE Act 1984, which stated that computer evidence should be subject to proof that it was, in fact, operating properly.

Court of Appeal judge Peter Fraser, who took charge of the 2018 High Court battle between the Post Office and 555 former subpostmasters, which focused on the reliability of the Post Office Horizon system, was speaking at a recent Inner Temple talk, titled: The use of electronic evidence in the law.

He said the Law Commission presumption, which found its way into statute nearly 30 years ago, needs to be replaced, but its replacement is not a simple choice. “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,” he told the audience.

“There needs to be some flexibility in how we approach it because the current complexity is just going to get more and more marked,” added Fraser.

In light of the abuse of the system on the part of the Post Office, the organisation has changed its tune. In a recent Post Office scandal public inquiry hearing, the Post Office’s head of investigations, John Bartlett, suggested there could be a return to the days of Section 69.

During the hearing, Bartlett, a former police officer, said: “I think one of the outcomes of the scandal is that, for all cases, irrespective of whether they’re Post Office derived, linked or otherwise, law enforcement are going to have to do those verification exercises, assurance exercises, on digital data, because I think the scandal has shown that the assumption made that systems could be relied upon has changed.

“I remember when I first joined the police, there was no natural assumption that a computer system was functioning and workable, and we had to do what were called system statements. I think one outcome of the scandal is that law enforcement will end up going back [to that]. So, I don’t think it will be particular to us.”

Simply putting Section 69 back in place as a reaction to what happened at the Post Office would be greatly over simple – it really needs a much more complicated response

This approach was discarded as a possibility during the Inner Temple talk.

One attendee, who had experience working with the previous rules on computer evidence, said: “Simply putting Section 69 back in place as a reaction to what happened at the Post Office would be greatly over simple and it really needs a much more complicated response.”

Judge Fraser agreed: “That is exactly right. In the 1990s, the systems were so simple and the presumption was necessary.”

“Maybe the solution going forward is not a one-size-fits-all answer and there should be a different response for complex systems and simple systems,” he added.

Stephen Murdoch, a professor of security engineering in the Computer Science Department at University College London, also spoke at the Inner Temple event. He agreed that something needs to change, but not a return to pre-1999 rules.

He told the audience: “I think something should be changed, but I don’t think Section 69 should be put back. For one, all it really required was that someone assert that the computer system is working properly, and in many cases, there were plenty of people willing to make that statement when they either didn’t know whether it went wrong, or they were unwilling to say despite knowing it’s incorrect.

“What I would far prefer is that all testing documents should be made available to the opposing side and the subject of challenge. That would hopefully give the court a better basis for deciding how much weight to put on the reliance of that computer evidence.”

The Post Office scandal has pointed a spotlight at the use of computer evidence in court. Since Judge Fraser’s Horizon trial judgment in December 2019, and as a direct result of it, subpostmasters have successfully appealed their wrongful convictions, many of which have been overturned.

From 2021 to early 2024, more than 100 former subpostmasters and branch staff had seen their convictions overturned on appeal, and following ITV’s dramatisation of the Post Office scandal and subsequent national outrage, government legislation has been introduced to overturn the wrongful convictions of hundreds of former subpostmasters who were prosecuted based on evidence from the Post Office Horizon system.

During prosecutions of subpostmasters between 2000 and 2015, the Post Office repeatedly stated in court that the system was robust. Its expert witnesses did the same. But in his damning 2019 judgment, Fraser likened the Post Office’s claim that Horizon was robust to claims made by flat-Earthers. He said the Post Office had exhibited “a simple institutional obstinacy or refusal to consider any possible alternatives to their view of Horizon, which was maintained regardless of the weight of factual evidence to the contrary”. 

Fraser added: “That approach by the Post Office was continued, even though now there is also considerable expert evidence to the contrary, and much of it agreed expert evidence on the existence of numerous bugs. It amounts to the 21st century equivalent of maintaining that the Earth is flat.”

Despite the lack of integrity of Horizon data, the rule change in 1999 made it easier for the Post Office, through its private prosecution powers, to prosecute subpostmasters for financial crimes when there were accounting shortfalls, based on data from the Horizon system.

In 1995, the Post Office made clear its support for a change in UK law regarding computer evidence during a Law Commission consultation on a proposed law change, claiming the previous rules were making prosecution “onerous”.

In a response to a Law Commission request for feedback on proposals to change the rules, the Post Office 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.”

The full Inner Temple talk can be viewed here: The use of electronic evidence in the law.

The Post Office scandal was first exposed by Computer Weekly in 2009, revealing the stories of seven subpostmasters and the problems they suffered due to Horizon accounting software, which led to the most widespread miscarriage of justice in British history (see below timeline of Computer Weekly articles about the scandal since 2009).