Post Office supported 1999 law change that eased prosecutions using computer evidence

The Post Office supported a 1999 change in the way UK law regarded computer evidence, which subsequently made it easier to prosecute subpostmasters for theft and false accounting. Without that law change, it would have been harder for the organisation to use its prosecutorial powers to convict the subpostmasters who - more than 20 years later - had those convictions overturned due to computer flaws.

In 1995 the Law Commission held a consultation, which sought feedback from prosecuting organisations, on a proposal to change the rules on the use of computer evidence in court.

At the time, Section 69 of the 1984 Police and Criminal Evidence Act (PACE) stated that computer-based evidence should be subject to proof that the computer system was operating properly. The Post Office, which has private prosecution powers, said this was "somewhat onerous" when prosecuting people charged with crimes, such as the subpostmasters that run and own its branches.

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 new law was adopted in 1999 and coincided with the introduction of the Horizon IT system, which automated accounting at thousands of Post Office branches.

In a reply to the consultation, which was revealed in response to a freedom of information (FoI) request made by computer scientist Steven Murdoch at University College London, the Post Office criminal law division said the rule at the time was “somewhat onerous from a prosecution viewpoint.”

The Post Office respondent wrote: “I consider that computer evidence is, in principal, 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.”

It wasn’t long after the introduction of Horizon that subpostmasters began to report unexplained accounting shortfalls. These business people, who own and run Post Office branches, were deemed responsible for the losses if they could not prove otherwise.

Between the launch of Horizon in 2000 and 2015, the Post Office used its private prosecution powers to prosecute 736 subpostmasters for financial crimes. Many of these suspected and claimed that Horizon was to blame for unexplained accounting shortfalls. The Post Office consistently denied that Horizon could be to blame and prosecuted the subpostmasters - thanks to the changes in the law around computer-based evidence, which presumed Horizon was working unless proved otherwise. Some people went to jail and many have lived with criminal records ever since.

In its response to the 1995 Law Commission consultation, revealed for the first time through FoI, the Post Office wrote: “In the event of a subpostmaster being prosecuted for theft or false accounting, the Post Office may need to rely upon the computerised accounting records. The subpostmaster is frequently the only person who can give the evidence required by Section 69 of [PACE]. In the absence of admissions or other direct evidence the Post Office may not be able to prove the case solely on the ground of being unable to satisfy the technical requirements of Section 69 of [PACE].”

In 2009, a Computer Weekly investigation first revealed that subpostmasters were being blamed for unexplained accounting shortfalls. Ten years later, a group of 555 subpostmasters won a High Court group litigation which proved that errors in the Horizon system could cause unexplained losses, and that the Post Office was aware of the bugs in the system.

Since then 72 subpostmasters who were convicted as a result of Post Office prosecutions have had their convictions overturned, some of them after 20 years. Many more are expected to follow.

In the latest group of subpostmasters that had wrongful convictions overturned, it emerged in court that the Post Office offered some of those who were charged with theft, the option to plead guilty to the lesser offence of false accounting, but only if they did not mention their suspicions that problems with Horizon were causing the shortfalls.