◆Part 1: The System
Post Office Counters Limited began rolling out the Horizon IT System to branches and Crown Offices in about September 1999. By about the end of 2001 or early 2002 the roll out was complete. Since then every branch and Crown Office has used a version of Horizon as an integral part of its business.
The first version, now usually called Legacy Horizon, was developed for and supplied to the Post Office by ICL Pathway Limited. That company became part of Fujitsu. Legacy Horizon ran until 2010. In 2010 it was replaced by Horizon Online, also supplied by Fujitsu.
Horizon recorded transactions and calculated how much cash and stock a branch should hold. Postmasters had to declare cash daily and complete a Branch Trading Statement at the end of each trading period. There was no facility inside Horizon for a postmaster to dispute the figures. Apparent shortfalls had to be made good.
◆Part 2: The Fiction
Sir Wyn Williams chaired the statutory Post Office Horizon IT Inquiry. Volume 1 of his Final Report was presented to Parliament on 8 July 2025 as HC 1119.
He found that prior to roll out some Fujitsu employees had discovered that Legacy Horizon was capable of producing false data. It could show losses or gains in branch accounts that were illusory rather than real. That state of affairs came from bugs, errors and defects.
Williams wrote that a number of senior, and not so senior, employees of the Post Office knew or, at the very least, should have known that Legacy Horizon was capable of that error. Yet for all practical purposes, throughout the lifetime of Legacy Horizon, the Post Office maintained the fiction that its data was always accurate.
Horizon Online was also afflicted by bugs, errors and defects that showed illusory gains and losses. Williams was satisfied that a number of employees of Fujitsu and the Post Office knew that this was so.
◆Part 3: The Prosecutions
Between 2000 and the autumn of 2013 the Post Office prosecuted postmasters and others who worked in branches and Crown Offices in England and Wales in reliance upon accounting data produced by Horizon. The data was used to prove that actual losses had occurred which could only be explained by theft, false accounting or fraud.
In Northern Ireland and Scotland the prosecuting authorities brought dishonesty prosecutions in the same period and for some years beyond 2013. In each, or at least most, of those cases they relied upon Horizon data. Across the United Kingdom the Post Office and the prosecuting authorities asserted that the data was wholly reliable.
Williams found it difficult to be precise about totals. On the evidence available to him he found it likely that approximately 1000 persons were prosecuted and convicted throughout the United Kingdom based on Horizon evidence. Evidence before the Inquiry also suggested that somewhere between 50 and 60 people were prosecuted but not convicted.
Thousands of postmasters were suspended on account of apparent shortfalls. A very significant number then had their contracts terminated.
◆Part 4: The Bugs
In 2019 Mr Justice Fraser tried civil group litigation between claimants representing about 580 postmasters and Post Office Limited. In Bates and Others v Post Office Limited (No.6: Horizon Issues) [2019] EWHC 3408 (QB) he found numerous bugs, errors or defects capable of causing, and that did cause, shortfalls.
He found 25 different bugs with the potential to impact branch accounts. Evidence of actual lasting impact existed for 22 of them. Horizon itself did not alert postmasters to those bugs, errors or defects.
Fraser J found that Fujitsu could insert, inject, edit or delete transaction data or data in branch accounts, implement fixes that could affect that data, or rebuild branch transaction data, all without the knowledge or consent of the postmaster. If Fujitsu injected a transaction into a branch account, this would look as though the postmaster had done it.
He concluded that Legacy Horizon was not remotely robust. Horizon Online was slightly more robust but still had a significant number of bugs, errors and defects, particularly between 2010 and 2015.
◆Part 5: The Court
On 23 April 2021 the Court of Appeal Criminal Division handed down Josephine Hamilton and Others v Post Office Limited [2021] EWCA Crim 577. Forty two former postmasters had been prosecuted by Post Office Limited between 2003 and 2013 and convicted of crimes of dishonesty. The Criminal Cases Review Commission referred their cases.
The Court found that throughout the relevant period there were significant problems with Horizon. Those problems gave rise to a material risk that an apparent shortfall did not reflect missing cash or stock but was caused by a bug, error or defect.
The Court held that the Post Office knew there were serious issues about the reliability of Horizon. It had a clear duty to investigate reasonable lines of enquiry and to disclose anything that might undermine its case. It does not appear that the Post Office adequately considered or made relevant disclosure of Horizon problems in any of those cases. It consistently asserted that Horizon was robust and reliable.
The Court said those concerned with prosecutions effectively steamrolled over any postmaster who sought to challenge the accuracy of the data. 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.
Thirty nine of the forty two appeals were allowed. All of those convictions were quashed. Three appeals failed because the Court found that the reliability of Horizon data was not essential to those prosecution cases.
◆Part 6: The Advice
Documents disclosed in the Court of Appeal appeals included written advice given to the Post Office in 2013 by barrister Simon Clarke.
In advice dated 15 July 2013 Clarke recorded that successful prosecution almost always depended on proper analysis of Horizon data, and that the integrity of Horizon had to be demonstrably robust. He stated that a Fujitsu employee, Gareth Jenkins, had provided statements attesting to Horizon integrity while aware of at least two bugs affecting Horizon Online, and had failed to mention them. Clarke advised that material which should have been disclosed was not disclosed, placing the Post Office in breach of its duty as a prosecutor.
In further advice dated 2 August 2013 Clarke recorded that he had been told minutes of a Horizon related conference call had been destroyed, with the word shredded conveyed to him, and that advice had been given inside the Post Office that if material was not minuted it was not in the public domain and therefore not disclosable. Clarke wrote that any such view was wrong in law and principle for a prosecutor.
Earlier, in August 2010, Rod Ismay, then Head of Product and Branch Accounting, prepared a report titled Horizon Response to Challenges Regarding Systems Integrity. The Court of Appeal recorded that the report warned that any investigation would need to be disclosed in court, that any perception the Post Office doubted its own systems would mean all criminal prosecutions would have to be stayed, and that it would raise a question for the Court of Appeal over past prosecutions and imprisonments.
◆Part 7: The Record
On 24 May 2024 the Post Office (Horizon System) Offences Act 2024 came into force. It quashed qualifying convictions in England and Wales that met statutory conditions tied to Horizon and Post Office work.
Ministry of Justice management information published on 2 July 2026 recorded that as of 30 June 2026, among Post Office, Crown Prosecution Service and self identified cases assessed in England and Wales, 610 individuals had been identified as having at least one conviction quashed by the Act. The total number of convictions identified as quashed by the Act in that table was 2228. Separate Royal Mail Group cases in the same publication added further assessments.
Williams Volume 1 addressed human impact and redress. He wrote that people held responsible for illusory losses were victims of wholly unacceptable behaviour by individuals employed by or associated with the Post Office and Fujitsu, and by those institutions.
The Inquiry Report, the High Court Horizon Issues judgment, and the Court of Appeal judgment stand as the primary record of what the system did, what the Post Office asserted, and what the documents showed.
Post Office Horizon IT Inquiry. Final Report Volume 1. Sir Wyn Williams. HC 1119. Presented to Parliament 8 July 2025. Bates and Others v Post Office Limited (No.6: Horizon Issues) [2019] EWHC 3408 (QB). Josephine Hamilton and Others v Post Office Limited [2021] EWCA Crim 577. Court of Appeal Criminal Division. 23 April 2021. Post Office (Horizon System) Offences Act 2024. Ministry of Justice. Quashed convictions management information: 2 July 2026.