The Horizon Appeal - Hamilton and others v Post Office Ltd.

23 April 2021


Hamilton and others v Post Office Ltd  [2021] EWCA Crim 577

Between 2003 and 2013 42 sub-postmasters and sub-postmistresses (“SPMs”) were prosecuted by the Post Office for various crimes of dishonesty. They were convicted.

The cases against each of them relied upon evidence from a computerised accounting system called “Horizon” which was used by the Post Office at that time. Every transaction at each Post Office branch had to be entered into Horizon. Horizon automatically generated regular reports about how much cash and stock the branch should have on hand. The prosecutions in this case related to times when there was a shortfall between the amount of cash that Horizon said the branch should have, and the amount of cash actually at branch.

Horizon was piloted in 1999 and rolled out in full in 2000. From the earliest days of its use, SPMs expressed concerns about its reliability and its accuracy. Many reported discrepancies and shortfalls that they considered were faults in Horizon, not faults in their own work. The Post Office maintained that Horizon was reliable, and therefore that any discrepancy or shortfall was the fault of the employees, not the system.

The Post Office’s general position was that where there was a shortfall, the SPMs were personally liable for paying back the difference. Where the Post Office believed the shortfall had come about dishonestly, they launched their own prosecutions against their employees. The criminal cases against the SPMs proceeded on the basis that Horizon was reliable. Some of the SPMs pleaded guilty, the rest were found guilty.

The Civil Case

Aside from the criminal case, many SPMs had been forced to pay their own money to the Post Office where Horizon had shown a shortfall. In 2017 a group of hundreds of SPMs launched a civil action against the Post Office, alleging that Horizon was unreliable. This was settled in 2019 shortly before Fraser J gave judgment. Fraser J found that there were numerous bugs, errors and defects in Horizon which were capable of causing (and did cause) shortfalls in post office branches. Further, a number of these bugs were not communicated to affected SPMs by the Post Office. In summary, Fraser J found that Horizon was “not remotely robust”.

The Appeal and Post Office’s Response

The Criminal Cases Review Commission considered the 42 SPM convictions in light of Fraser J’s findings and appeals were launched.

For each of the SPMs there were two grounds of appeal. The first was that the reliability of Horizon data was essential to the prosecution and, in the light of all the evidence including Fraser J’s findings in the High Court, it was not possible for them to have had fair trials. The second ground, going further still, was that the evidence shows that it had been an affront to the public conscience for the SPMs to face prosecution.

The Post Office conceded the first ground of appeal for all but three of the appellants. They accepted that the Horizon data was vital to the prosecutions and, consequently, that the trials were not fair. They denied Ground 2, that the prosecutions were an affront to public conscience.


The Court of Appeal found that the Post Office had known that there were serious issues about the reliability of Horizon. They had a clear duty to investigate all reasonable lines of enquiry and to disclose anything that might undermine their prosecution case. However, despite advice from external counsel, the Court of Appeal found that it did not appear that the Post Office adequately considered or disclosed the problems or concerns about Horizon in any of the prosecutions. Further, it did not appear that the Post Office made any attempt to investigate any of the SPMs’ assertions that Horizon was unreliable. The Court concluded that “the consistent failure of [the Post Office] to be open and honest about the issues affecting Horizon can in our view only be explained by a strong reluctance to say or do anything which might lead to other SPMs knowing about those issues. Those concerned with prosecutions of SPMs clearly wished to be able to maintain the assertion that Horizon data was accurate, and effectively steamrolled over any SPM who sought to challenge its accuracy.”

In 39 of the 42 cases (the same 39 conceded by Post Office), the reliability of Horizon was essential to their case. In summary: there was no basis for prosecution as Horizon was not reliable. In the three remaining cases, Horizon was not central to the prosecution case.

In respect of Ground 2, the court concluded that Post Office’s failures of investigation and disclosure were so egregious as to make the prosecution of any of the “Horizon cases” an affront to the conscience of the court. Post Office had effectively reversed the burden of proof: by treating Horizon as reliable, despite all of the suspicions to the contrary, they had effectively reversed the burden of proof, forcing defendants to prove that no loss had occurred. The defendants had been denied disclosure of the material indicating that Horizon was not reliable, and so were unable to defend themselves properly.

Therefore, the 39 appeals that succeeded on Ground 1 also succeeded on Ground 2. The three appeals which were not considered to be reliant upon Horizon were each dismissed.


There is a tendency both in the civil and criminal courts to assume that computers are reliable. Indeed, in many areas of law such assumptions are hardcoded in legislation as rebuttable presumptions. This is a stark warning to be cautious even when a case relying solely upon computer evidence or records looks bleak.

This case was, of course, compounded by what appears to be an intentional decision by the prosecutor not to disclose material undermining their case. Indeed, when considering a schedule of unused material, it was found that on one occasion the Disclosure Officer had marked an item as sensitive and refused to disclose it with the commend “Could be used as mitigation, ie to blame Horizon system for the loss”. This was a serious error in the Post Office’s approach to disclosure.

For those who prosecute, it is a vital reminder of the responsibilities of full, candid disclosure. External counsel advised Post Office in clear and firm terms about their disclosure obligations, but it does not appear that his advice was followed. The Court, however, went a step further, commenting that in light of over a decade of complaints about Horizon it was “extraordinary that it was necessary for Mr Clarke [the external counsel] to advise in those terms.”.

For those who defend, especially against large organisations whose primary focus is not prosecution, or even against private prosecutions, it is a reminder to keep a very careful eye on their approach and attitude to disclosure. In this Horizon appeal, 39 people, all of good character, were convicted of serious crimes in the most unfair of circumstances; we must not forget that three of those 39 died before their names were cleared.

Ben Rowe.
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