An independent investigation into the integrity of the current version of the controversial Post Office Horizon system and its ability to identify discrepancies will be completed in the autumn.

The controversial system, which is at the centre of the Post Office scandal, could be used in branches for another eight years while an off-the-shelf alternative is sought.

The third-party review of the Fujitsu-supplied Horizon system, which has been in use in branches since 1999, was revealed in November 2024 during a Post Office scandal public inquiry hearing. When giving evidence, Lorna Gratton, the civil servant who sits as a non-executive director of the Post Office on behalf of the government, said there had been a discussion over the current reliability of the Horizon system.

Computer Weekly revealed the review was being carried out by investigation firm Kroll.

The Post Office said the investigation includes an assessment of the integrity of the data in Horizon and a review of the Post Office’s discrepancy identification and resolution process. It said it is a response to feedback from subpostmasters, including evidence heard at the public inquiry.

The Horizon system is at the centre of the widest miscarriage of justice in UK history. Thousands of subpostmasters were blamed for accounting discrepancies caused by computer errors, with many covering phantom shortfalls by repaying huge sums of money to the Post Office. Many subpostmasters were bankrupted, put out of business and wrongly convicted of financial crimes.

A September 2024 report by the public inquiry raised concerns about the current version of the controversial system. It found that 57% of current subpostmasters have experienced unexplained shortfalls.

Of those surveyed, almost all 92% had encountered an issue with the Horizon IT system in the 12 months prior to the survey, with screen freezes and lost connectivity being the most common problems. 

Three-quarters said they have used their own branch money to cover discrepancies or resolved the issue themselves.

During a public inquiry hearing last year, Fujitsu’s European boss, Paul Patterson, admitted there are bugs and errors in the Horizon software, but said blaming the application for all issues is the result of too “narrow” a definition of Horizon. He said screen freezes could be caused by multiple things, and connectivity is provided by other suppliers.

He also said systems that connect to Horizon, such as POLSAP, the Post Office’s SAP platform, should also be examined when issues occur.

The system is set to remain in branches for years to come. According to a public sector procurement notice, the Post Office will finally replace the controversial Horizon system with an off-the-shelf alternative, but this might not be complete until 2033.

Fujitsu’s involvement, however, will end next year, with the Post Office looking for a new supplier to take over Horizon’s operation while it finds a replacement system. In total, this will cost taxpayers about £500m.

The government has so far paid out £1bn in compensation to those affected by the scandal.

Separately, the Post Office said there is no update on its project to migrate Horizon data from Fujitsu infrastructure to its own. The migration was paused on request of the Metropolitan Police due to its “concerns”.

Computer Weekly first exposed the scandal in 2009, revealing the stories of seven subpostmasters and the problems they suffered due to the Horizon accounting software, which led to the most widespread miscarriage of justice in British history (see below timeline of Computer Weekly articles).

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