When a court ruled the computer was wrong: the Post Office Horizon scandal, 39 convictions quashed and an Act of Parliament to overturn the rest

Verification status: PENDING — every quoted figure is taken verbatim from the primary court and legislative record (the Court of Appeal’s published judgment, the High Court’s Horizon Issues judgment, the Post Office (Horizon System) Offences Act 2024) and official public bodies (the CCRC and GOV.UK). Graded against the public record and awaiting the owner’s validation.

The problem

The Post Office did not so much buy a fraud detector as trust a computer’s arithmetic over its own people. From 2000 it rolled out Horizon, a computerised branch-accounting and electronic point-of-sale system operated on its behalf by Fujitsu, across branch post offices; “the IT system used in both independent and Crown Post Offices was called Horizon, operated by Fujitsu on behalf of Post Office” (source). When Horizon reported a shortfall in a branch’s accounts, the Post Office treated that figure as proof that cash was missing, and it prosecuted the people behind the counter. Over 15 years “the Post Office used its private investigation and prosecution powers to prosecute more than 700 sub-postmasters” (source). As the Court of Appeal later put it, the subpostmasters “were prosecuted and convicted on the basis that Horizon was reliable, when in fact it was not” (source). This is a verified negative: the findings were made by courts and Parliament, not a vendor, and they were made about a live automated system that had been treated as beyond question.

What the courts found

The reliability of the system was tested first in civil litigation. In the December 2019 “Horizon Issues” judgment, Bates and Others v Post Office Ltd [2019] EWHC 3408 (QB), Mr Justice Fraser found Horizon was not the robust system the Post Office claimed but “was prone to bugs, errors and defects” (source). The Court of Appeal restated that finding in its own words and made its significance plain: “throughout the relevant period there were significant problems with Horizon, which gave rise to a material risk that an apparent shortfall in the accounts of a branch post office did not in fact reflect missing cash or stock, but was caused by one of the bugs, errors or defects in Horizon” (source). Against that, the Post Office’s posture had been categorical: it “consistently asserted that Horizon was robust and reliable” (source).

On 23 April 2021 the criminal reckoning followed. In Josephine Hamilton and Others v Post Office Limited [2021] EWCA Crim 577 the Court of Appeal heard the appeals of 42 former subpostmasters, all referred by the Criminal Cases Review Commission, and concluded that the 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” (source). The consequences of trusting the machine over the person were spelled out: “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” (source). The court quashed the convictions: it summarised “the facts of the 39 individual cases [paras 140-356], concluding that all the convictions are unsafe” (source), dismissing only the three appeals in which the reliability of Horizon data had not been essential to the prosecution.

The outcome

The courts could only reach the cases in front of them, and they were a fraction of the total. The Criminal Cases Review Commission “referred 77 ‘Horizon’ convictions to the appeal courts” (source), a number dwarfed by the more than 700 people the Post Office had prosecuted. To clear the backlog, Parliament took the rare step of overturning convictions by statute. The Post Office (Horizon System) Offences Act 2024 is, in its own long title, “An Act to provide for the quashing of convictions in England and Wales and Northern Ireland for certain offences alleged to have been committed while the Horizon system was in use by the Post Office” (source), and section 1 does exactly that: “Every conviction to which this Act applies is quashed on the coming into force of this Act” (source). The Act received Royal Assent on 24 May 2024 (source). Redress followed on a scale that matched the failure: by 9 June 2025 the government reported that “more than £1 billion has been paid out to over 7,300 postmasters affected by the Horizon IT scandal” (source).

Weakest load-bearing link. The two critical claims here are the High Court finding on Horizon and the Court of Appeal quashing the convictions, and both rest on Tier-1 primary documents. The honest limit is one of presentation rather than substance: the verbatim Court of Appeal wording is quoted from the Judiciary’s own court-published summary of [2021] EWCA Crim 577, which states that it “does not form part of the reasons for the decision” and that “the full judgment of the court is the only authoritative document.” The full judgment PDF and the 2019 Bates judgment PDF are both archived here, but could not be machine-extracted this session (the box has no PDF text-extraction tooling and BAILII returns 403 to the fetcher), so Fraser J’s finding is carried through the Court of Appeal’s verbatim restatement rather than lifted directly from the 2019 judgment. The substance is unaffected: the summary is the court’s own publication, and the same findings are independently anchored by the CCRC’s official account and the primary legislation. A second, deliberate limit: the exact amount of the December 2019 civil settlement is not stated here, because reachable sources conflicted on the figure, so it is left out rather than merged or rounded.

How this was verified. Method: every load-bearing sentence is quoted verbatim from a primary or official source fetched this session and saved to sources/. The 42 appeals heard, the 39 convictions quashed, “all the convictions are unsafe”, “an affront to the conscience of the court”, the “prosecuted and convicted on the basis that Horizon was reliable, when in fact it was not” framing, and the verbatim restatement of Fraser J’s finding (“bugs, errors or defects in Horizon”) come from the Judiciary of England and Wales’ court-published summary of Josephine Hamilton and Others v Post Office Limited [2021] EWCA Crim 577, 23 April 2021 (Tier 1; sources/hamilton-summary-2021.pdf, with the full judgment archived at sources/hamilton-judgment-2021.pdf). The underlying High Court finding is Bates and Others v Post Office Ltd (No 6: Horizon Issues) [2019] EWHC 3408 (QB), Mr Justice Fraser, 16 December 2019 (Tier 1; PDF archived at sources/bates-horizon-issues-2019.pdf). The scale figures (“more than 700 sub-postmasters” prosecuted over 15 years, 77 CCRC referrals, Horizon operated by Fujitsu) are from the Criminal Cases Review Commission (Tier 2, an official public body; sources/ccrc-post-office-horizon-cases.txt). The en-masse quashing and long title are quoted from the Post Office (Horizon System) Offences Act 2024 (2024 c. 14), legislation.gov.uk (Tier 1; sources/legislation-po-horizon-offences-act-2024.txt), Royal Assent 24 May 2024. The redress figure is from GOV.UK, “Compensation to postmasters reaches £1 billion milestone”, 9 June 2025 (Tier 1; sources/govuk-compensation-1-billion-milestone.txt). No confirmation was sought from the Post Office, Fujitsu or any party; only already-public records are used. Checked 2026-09-02 (round 1).

Related case files

• The Hague District Court declares the Dutch SyRI welfare-fraud algorithm unlawful — another court overturning an automated government system that was trusted over the people it profiled, on the public record rather than by a vendor’s account.
• Michigan’s MiDAS fraud algorithm wrongly accused tens of thousands — the closest US parallel: an automated system’s outputs treated as proof of fraud, mass false accusations, and a due-process reckoning in the courts.
• Australia’s Robodebt automated income-averaging scheme found unlawful — an automated debt-raising system whose figures were presumed correct, unwound through a royal commission and a class-action settlement.
• The Dutch DPA fines the tax authority €2.75M for unlawful algorithmic risk profiling — the same pattern priced by a regulator rather than a criminal court: an automated fraud-scoring system that wrongly branded ordinary people.

Sources

1. Judiciary of England and Wales (Court of Appeal, Criminal Division) · Josephine Hamilton and Others v Post Office Limited [2021] EWCA Crim 577 — court summary · 23 April 2021 · https://www.judiciary.uk/wp-content/uploads/2022/07/Hamilton-Others-v-Post-Office-summary-230421.pdf — Tier 1 (the court’s own published summary of the decision: 42 appeals heard, 39 convictions quashed as unsafe, prosecutions an abuse of process on both grounds, the Post Office’s insistence that Horizon was “robust and reliable”, and the verbatim restatement of Fraser J’s “bugs, errors or defects” finding. Downloaded and read locally; full judgment PDF also archived at sources/hamilton-judgment-2021.pdf).
2. High Court of Justice, Queen’s Bench Division · Bates and Others v Post Office Ltd (No 6: Horizon Issues) [2019] EWHC 3408 (QB) · Mr Justice Fraser, 16 December 2019 · https://www.bailii.org/ew/cases/EWHC/QB/2019/3408.html — Tier 1 (the primary “Horizon Issues” judgment finding Horizon prone to bugs, errors and defects; PDF archived at sources/bates-horizon-issues-2019.pdf. Its finding is carried here via the Court of Appeal’s verbatim restatement in source 1, as the PDF could not be machine-extracted this session).
3. Criminal Cases Review Commission · Post Office ‘Horizon’ Cases · accessed 2 September 2026 · https://ccrc.gov.uk/post-office-horizon-cases/ — Tier 2 (official public body corroborating that the Post Office prosecuted “more than 700 sub-postmasters” over 15 years, that the CCRC referred 77 Horizon convictions to the appeal courts, and that Horizon was operated by Fujitsu; saved to sources/ccrc-post-office-horizon-cases.txt).
4. legislation.gov.uk (The National Archives) · Post Office (Horizon System) Offences Act 2024 (2024 c. 14) — long title and section 1 · Royal Assent 24 May 2024 · https://www.legislation.gov.uk/ukpga/2024/14 — Tier 1 (primary legislation quashing the in-scope Horizon convictions en masse: “Every conviction to which this Act applies is quashed on the coming into force of this Act”; saved to sources/legislation-po-horizon-offences-act-2024.txt).
5. GOV.UK (Department for Business and Trade) · Compensation to postmasters reaches £1 billion milestone · 9 June 2025 · https://www.gov.uk/government/news/compensation-to-postmasters-reaches-1-billion-milestone — Tier 1 (UK government statement that more than £1 billion has been paid to over 7,300 affected postmasters; saved to sources/govuk-compensation-1-billion-milestone.txt).