How a single software system triggered the UK's largest series of wrongful convictions
Between 1999 and 2015, hundreds of innocent people were prosecuted for crimes they did not commit. The culprit was not a person, but a series of computer bugs within the Horizon system that misreported financial data, leaving sub-postmasters with no way to prove their innocence.
The Post Office Horizon scandal stands as the most widespread miscarriage of and justice seen by the Criminal Cases Review Commission (CCRC). The crisis centered on the Horizon IT system, operated by Fujitsu, which was piloted in 1996 and rolled out across the UK network in 1999. The system was designed to track transactions and calculate the expected levels of cash and stock for individual branches. However, significant bugs caused the software to misreport figures, often involving substantial sums of money.
The mechanism of the injustice was embedded in the system's design and the Post Office's rigid protocols. Sub-postmasters were required to enter a daily cash declaration, but if a discrepancy appeared between physical cash and Horizon's generated figures at the end of a trading period, the sub-postmaster was contractually obligated to make up the difference. Crucially, the Horizon interface offered no way to dispute these figures. Because the system replaced paper-based accounting, there was no traditional chain of evidence for sub-postmasters to use to challenge the errors.
The human cost was immense. For 15 years, the Post Office used its investigation and prosecution powers to target more than 700 sub-postmasters, leading to prison sentences, criminal records, and bankruptcies. Even as problems were reported by Computer Weekly in 2009, call centre staff were instructed to tell victims they were the only ones experiencing issues. While the Justice for Subpostmasters Alliance (JFSA) formed to campaign for justice, it was not until December 2019, following High Court trials, that the Post Office finally conceded and settled with 555 sub-postmasters involved in group litigation.
Source: Post Office Horizon Scandal - Computerphile
