Accounting System Credential Authority Failure Through IT System Output Treated as Evidence of Criminal Conduct Without Independent Verification of System Accuracy at UK Post Office Horizon

The Post Office operates through a network of branches, most of which are run by sub-postmasters — independent contractors who manage the branch and are contractually responsible for any financial discrepancies in their accounts. In 1999, the Post Office began deploying the Horizon IT system across its branch network, replacing paper-based accounting. Horizon recorded all transactions — stamp sales, bill payments, banking services, benefits payments — and maintained the financial accounts for each branch. Under sub-postmasters' contracts, they were liable for any shortfall the system showed, regardless of the cause.

Almost immediately after Horizon's deployment, sub-postmasters began reporting unexplained discrepancies in their accounts. Branch balances showed shortfalls that operators could not explain or trace to any actual transaction. Sub-postmasters contacted the Post Office helpline reporting that the system appeared to be generating errors. They were told, consistently and repeatedly, that Horizon was robust, that no other branches were experiencing problems, and that the shortfall was their responsibility. Many sub-postmasters used personal savings or took out loans to cover the discrepancies. Those who could not or would not pay were investigated and prosecuted.

Over more than a decade, the Post Office prosecuted over 900 sub-postmasters for theft, fraud, and false accounting. The prosecutions relied substantially on Horizon's accounting data as the evidence of wrongdoing — the system showed a shortfall, therefore money was missing, therefore the sub-postmaster took it. Many defendants were convicted. Some were imprisoned. Others received suspended sentences, community service orders, or were bankrupted by the requirement to repay shortfalls that did not correspond to actual missing money. Lives, businesses, marriages, and reputations were destroyed. Some sub-postmasters died before their names were cleared.

A group litigation brought by 555 sub-postmasters against the Post Office, decided in 2019, found that Horizon contained bugs and defects capable of generating the unexplained shortfalls. The judge found that the Post Office had not been open about the system's problems and that Horizon was not as robust as the Post Office had maintained. Subsequent investigation revealed that Fujitsu staff had the ability to remotely access and alter branch-level data without the sub-postmaster's knowledge — a capability the Post Office had not disclosed to defendants or courts during prosecutions.

The Horizon system's output was treated as a reliable credential of financial reality. When the system showed a shortfall, the Post Office treated the shortfall as a fact — money was missing, and the person responsible for the branch was responsible for the missing money. The system's output was not independently verified against physical cash counts, transaction logs from counterparties, or other external records that would have confirmed whether the shortfall reflected actual missing money or a system error. The system said money was missing. The Post Office accepted what the system said.

When sub-postmasters reported that the system was generating errors, the Post Office's response was to assert the system's reliability rather than investigate its accuracy. The organization that owned the system and used its output to prosecute operators did not independently verify that the system was producing correct results. The legal presumption that computer evidence is reliable — a presumption embedded in English law — supported the prosecution's case. The defendants were required to prove the system was unreliable, despite having no access to Horizon's source code, no knowledge of its bugs, and no information about whether other branches were experiencing similar problems. The system existed. The system was certified. The system was treated as authoritative. The independent verification that the system was actually producing accurate outputs — especially when the people it accused said it was wrong — did not occur.

The Court of Appeal quashed 39 convictions in 2021. The Post Office Horizon IT Inquiry (Williams Inquiry) was established to investigate. In 2024, Parliament passed the Post Office (Horizon System) Offences Act, legislatively overturning all remaining Horizon-related convictions — an extraordinary step reflecting the scale and severity of the injustice. A compensation scheme was established. The inquiry continued examining the Post Office's knowledge of Horizon's defects and its decision to prosecute despite that knowledge. The case became the largest miscarriage of justice in British legal history.