The Post Office Horizon scandal: the law says computers are reliable

The Post Office Horizon scandal will probably have come to the attention of most lawyers over the last 12 months. There are a number of significant issues other than legal that surround the scandal, and Paul Marshall of Cornerstone Barristers, Gray’s Inn, succinctly summarises them in a lecture he gave at the University of Law on 3 June 2021.

This discussion considers what is, arguably, the underlying legal cause of the scandal – that computers are presumed to be reliable.

The circumstances leading to group action

A group of ex-sub-postmasters and sub-postmistresses formed the Justice For Subpostmasters Alliance (JFSA) in 2009 because of experiencing significant problems with how the Post Office dealt with apparent shortfalls in their accounts after the introduction of the Horizon IT system in 2000. Following years of campaigning with the support of many MPs, in 2012 the Post Office appointed Second Sight Support Services Limited, a firm of independent forensic accountants, to investigate the claims being made about the Horizon system.

In 2013 an Initial Complaint Review and Mediation Scheme was established to investigate individual cases. A Working Group, comprising of representatives from Second Sight, the Post Office and the JFSA was established with an independent chair. The Scheme closed to applicants after twelve weeks. On 9 April 2015, the Post Office ended the Scheme Working Group and terminated the contract with Second Sight, together with that of the independent Chairman.

The draft of Part Two of the Report by Second Sight was due to be released to the Working Group on 10 April 2015, but the action of the Post Office prevented this from taking place. The second part of the Second Sight Report eventually appeared on a journalists’ website.

The trials

In 2015, Freeths LLP agreed to represent those ex sub-postmasters and sub-postmistresses that wanted to take part in any legal action. Therium Group Holdings Limited funded the litigation. A Group Litigation Order was subsequently made on 22 March 2017 by Senior Master Fontaine, and approved by the President of the Queen’s Bench Division.

The first trial concerned the contractual position between the Post Office the sub-postmasters and sub-postmistresses. The judgment is in Bates v Post Office Ltd (No 3) [2019] EWHC 606 (QB).

The second trial, dealing with the Horizon software, took place between 11 March 2019 and 22 July 2019. Between the end of the second trial and the judgment, the parties sought mediation. An agreement was reached on 11 December 2019. The Confidential Settlement Deed (pdf) was eventually made public.

The judge handed down his judgment in the second trial on 16 December 2019 in Bates v The Post Office Ltd (No 6: Horizon Issues) Rev 1 [2019] EWHC 3408 (QB). When handing down his judgment, the judge indicated he:

“… had very grave concerns regarding the veracity of evidence given by Fujitsu employees in other courts in previous proceedings about the known existence of bugs, errors and defects in the Horizon system. These previous proceedings include the High Court in at least one civil case brought by the Post Office against a sub-postmaster and the Crown Court in a greater number of criminal cases, also brought by the Post office against sub-postmasters and sub-postmistresses.

After careful consideration, I have therefore decided, in the interests of justice, to send the papers in the case to the Director of Public Prosecutions, Mr Max Hill QC, so he may consider whether the matter to which I refer should be the subject of any prosecution.”

(The approved transcript of the proceedings by the judge on 16 December 2019 are to be published by the Digital Evidence and Electronic Signature Law Review in the autumn of 2021.)

The appeal

In 2015, the Criminal Case Review Commission (CCRC) began reviewing claims of wrongful prosecution for offences such as theft and false accounting, caused, the complaints alleged, as a result of problems with Horizon. The CCRC subsequently referred 47 Post Office cases on the abuse of process to the Court of Appeal. The hearing took place on 15 January 2021: R v Hamilton [2021] EWCA Crim 21. There were two grounds of appeal: that the defendants could not have a fair trial, and that his or her trial was an affront to the conscience of the court.

The Court of Appeal subsequently heard the appeal of 42 appellants in March 2021 and handed down judgment on 23 April 2021: Hamilton v Post Office Ltd [2021] EWCA Crim 577. The court reached three conclusions:

“First, disclosure by the Post Office was inadequate.

Second, the prosecutions were an affront to the conscience of the court (a rare finding) at [66].

Third, by constantly asserting that the Horizon system was ‘reliable’ [20] and [125], ‘accurate and reliable’ [68] or ‘robust and reliable’ [121], the Post Office effectively sought to reverse the burden of proof. In delivering the judgment of the court, Holroyde LJ said at [137]:

‘By representing Horizon as reliable, and refusing to countenance any suggestion to the contrary, POL effectively sought to reverse the burden of proof: it treated what was no more than a shortfall shown by an unreliable accounting system as an incontrovertible loss, and proceeded as if it were for the accused to prove that no such loss had occurred. Denied any disclosure of material capable of undermining the prosecution case, defendants were inevitably unable to discharge that improper burden.’”

Not only was it factually incorrect that the Horizon system reliable, but the failure to disclose relevant information meant ([137]):

“[the] defendants were inevitably unable to discharge that improper burden. As each prosecution proceeded to its successful conclusion the asserted reliability of Horizon was, on the face of it, reinforced. Defendants were prosecuted, convicted and sentenced on the basis that the Horizon data must be correct, and cash must therefore be missing, when in fact there could be no confidence as to that foundation.”


Many commentators have suggested that the main legal problem with the Post Office Horizon prosecutions was that the Post Office failed to disclose relevant documents about the Horizon system, and that this failure of disclosure was the principle cause of the scandal. This is partly true. For instance, in the case of Seema Misra, the defence made a number of requests for further disclosure of the computer system. This was refused four times:

  • The first application was before Mr Recorder Bruce on 10 March 2010 (Day 1 Monday 11 October 2010, 3C (Judge’s Ruling, Day 1 Monday 11 October 2010, 25, A-C)).
  • The second application was before HH Judge Critchlow on 7 May 2010 (Day 1 Monday 11 October 2010, 3G).
  • The third application was made before the trial judge (Day 1 Monday 11 October 2010, 15H-16H).
  • The fourth application was also made before the trial judge (Day 6, Monday 18 October 2010, 24H-25A).

The transcripts are available in the Digital Evidence and Electronic Signature Law Review (Vol 12, 2015).

The question that must be asked is why did three different judges refuse disclosure of relevant evidence? It is only possible to speculate about the answer. For instance, it is possible that the judges did not know much about electronic evidence and therefore did not realise the importance of the defence requests. Possibly the judges might have thought that because computers were deemed to be reliable, by making these applications, the defence was merely on a ‘fishing expedition’ that (according to the Post Office) cost the Post Office a great deal of unnecessary money – yet we know that a well-managed system has all the evidence required easily available and at very little cost, for which see: Paul Marshall, James Christie, Peter Bernard Ladkin, Bev Littlewood, Stephen Mason, Martin Newby, Jonathan Rogers, Harold Thimbleby, Martyn Thomas CBE, ‘Recommendations for the probity of computer evidence’, 18 Digital Evidence and Electronic Signature Law Review (2021) 18-26 (

On the presumption that computers are “reliable”

This topic is dealt with in depth in chapter 5 of Electronic Evidence and Electronic Signatures (5th edition, August 2021, IALS OBServing Law).

In 1997, the Law Commission proposed the repeal of s 69 of the Police and Criminal Evidence Act 1984 and a return to the common law presumption: “In the absence of evidence to the contrary, the courts will presume that mechanical instruments were in order at the material time.” (Evidence in Criminal Proceedings: Hearsay and Related Topics (1997), 13.13) The words “mechanical instruments” included computers and computer-like devices. The authors of the report did not produce any evidence to demonstrate that software code should benefit from this assertion. There was also no discussion of what is meant by “in order”. This is important, bearing in mind that the presumption is a presumption without the requirement of proof of a basic fact. The importance of the failure to justify that computers should benefit from this presumption has been clearly demonstrated in the Post Office Horizon scandal.

Arguably, the Post Office will not have taken such a cavalier attitude to responding to alleged shortfalls if it had been made – as it ought to have been – to reveal the errors of the Horizon IT system when prosecuting sub-postmasters and sub-postmistresses.

One example: how the lawyers were alleged to have dealt with one case

The journalist Nick Wallis, who has been reporting on this issue for over ten years, and who runs the website, has published the comments by Tim Brentnall, who took over the Post Office counter in his parents’ shop in Roch in Pembrokeshire. The website indicates that Tim was prosecuted for false accounting in 2010 over a discrepancy of £22,000. After he paid this money, the Post Office prosecuted him for false accounting. The comment on the website states that it did not “occur to Tim or his legal team to challenge the integrity of the Horizon data”. The website states that, “His solicitors and barrister advised him to plead guilty to stay out of jail. He did, and was given an 18 month suspended sentence with 200 hours community service.”

The solicitors and barrister are unknown, and it is not possible to clarify the comment attributed to their actions. However, regardless of what took place between Mr Brentnall and his lawyers, it appears, on the face of the story, that the lawyers certainly need to be made aware of electronic evidence.

Further developments

In October 2020 the government initiated a non-statutory inquiry into the scandal led by Sir Wyn Williams, a retired High Court judge. After sustained pressure from interested parties, in June 2021 the government agreed to convert the Inquiry into a statutory inquiry under the Inquiries Act 2005.

It is to be sincerely hoped that Sir Wyn Williams will consider this aspect of the scandal carefully.

Stephen Mason was called to the Bar by Middle Temple in 1988, and is an Associate Research Fellow at the Institute of Advanced Legal Studies in London. Email

He is the joint editor, with Professor Daniel Seng, of Electronic Evidence and Electronic Signatures (5th edition, August 2021, IALS OBServing Law).

He also founded the open source international journal Digital Evidence and Electronic Signature Law Review.

Image cc public domain.