Pressure is growing on the UK government to change a legal presumption that computer evidence is correct unless there is explicit evidence to the contrary. The issue has come to the fore again as the statutory public inquiry into the Post Office Horizon scandal, one of the biggest miscarriages of justice in UK history, progresses.

With the use of and reliability on computer systems accelerating even faster with the development and deployment of AI, the application of this basic principle has far-reaching effects.

In legal terms, a presumption affects the allocation of the burden of proof between parties in proceedings by doing away with the need to prove every item of evidence and saving time and expense in proving the obvious. Until 1999, Section 69 of the Police and Criminal Evidence Act 1984 (PACE) specified that computer evidence should be subject to proof that the computer system was operating properly.

But the UK Law Commission recommended a change and Section 69 of PACE was repealed by the Youth Justice and Criminal Evidence Act 1999, establishing the presumption that computer evidence was correct. There is a detailed explanation of the reasons for the change and the problems that gave rise to on University College London’s Bentham’s Gaze information security blog.

Post Office Horizon scandal

Focus on this legal presumption has sharpened as a result of the Post Office Horizon scandal, first revealed by UK trade publication Computer Weekly in 2009. Over a period of 15 years, 736 subpostmasters were prosecuted by the Post Office for fraud. The prosecutions were based on evidence from the Post Office’s Horizon retail and accounting system supplied by Fujitsu, but it has subsequently been established that the system was error-prone and figures provided by it should never have been used in court.

The case has been documented in depth by Computer Weekly in scores of articles since 2009. To date, hundreds of convictions have been declared unsafe and to have been obtained unlawfully, and 86 overturned. But the human cost of the prosecutions has been brutal, with subpostmasters and their families being subject to imprisonment, defamation, loss of livelihood, bankruptcy and in some cases the stress has led to divorce and suicide. The true extent of the miscarriage of justice is still unknown.

Last week, during a hearing at the public inquiry, a letter from the Post Office to the Law Commission written in 1995 in response to a request for feedback on changing the rules was referenced by barristers, bringing calls to change the legal presumption into the spotlight once more.

Computer evidence presumed accurate

In the letter the Post Office said that it would be difficult for it to prosecute unless computer evidence was presumed to be accurate. In 2019, the case Bates & Others v Post Office Ltd established that software errors and defects did exist in the Horizon system, and that they could cause apparent accounting discrepancies.

The inquiry has also heard that the Post Office had a policy of not sharing evidence of potential system failures from subpostmasters it was prosecuting. This policy continued at the same time as the Post Office continued to maintain the line in public that there were no system errors that could have caused the otherwise unexplained shortfalls. That line was stuck to until the Bates case in 2019.

The introduction of the presumption of computer system correctness has been described as “wrong-headed” and “fatuous nonsense” by barrister Paul Marshall, who represented subpostmasters who overturned convictions. He submitted a paper to the UK Minsitry of Justice at the request of MP Alex Chalk recommending that a party relying on electronic evidence should automatically be required to provide details of how their systems are professionally managed.

Legal opinion

But the UK government, which was the sole shareholder in the Post Office, has so far resisted calls for change, with then parliamentary under-secretary of state for justice James Cartlidge responding to calls for review by saying: “We have no plan to review the presumption, as it has wide application and is rebuttable if there is evidence to the contrary.”

A growing body of opinion believes the law should be changed, and that at the very least the Marshall recommendations should be adopted. That is partly because while the presumption may be rebuttable in principle, what happens in practice is that individuals rarely have the resources to do so.

The UK government appears to be leaving the next step to the inquiry, so progress towards change would need to come in a specific recommendation from the inquiry. In the meantime, mounting evidence at the hearings will keep the issue in the spotlight and increase the chances of public pressure being brought to bear.

This has clear implications for discussions over the application and regulation of AI that are currently prevalent. Once again, it is the way in which humans use technology, rather than the technology itself, that is fundamental.