BCS demands reform to rules on computer evidence following Post Office Horizon scandal revelations

by Joseph K. Clark

BCS, The Chartered Institute for IT, has called for reform to how computer evidence is treated in courts following the wrongful prosecutions of sub-post managers. The IT professional body also wants the government’s statutory inquiry into the scandal to consider the current rules on digital evidence. “We are calling for a swift review of how the courts treat computer evidence,” said Paul Fletcher, CEO at the BCS.

From 2000 for 15 years, the Post Office blamed sub-post managers for unexplained shortfalls in their accounts. It used evidence from the Horizon retail and accounting system, which sub-post managers use in branches, to prosecute those that could not account for shortfalls. But the subpostmasters claimed the Horizon system was to blame because the shortfalls only started after Horizon was introduced in 1999/2000. They were proved right in a High Court group litigation order in 2019.

The Post Office could prosecute the sub-post managers using Horizon evidence, despite its errors, because of a change in court rules concerning computer evidence, which came into force in 1999. Based on Horizon data, 736 sub-post managers were prosecuted, with some sent to prison, many made bankrupt, and families ruined. A total of 47 former sub-post managers recently had their criminal records quashed after years of campaigning, and hundreds more could follow.


Before then, prosecutors who relied on digital evidence in court had to prove that the computer system had worked as it should. However, the year’s change to the rules meant it was now presumed that the computer system worked correctly unless there was explicit evidence. Fletcher said this should not be the case, and organizations that relied on data from computer systems to support prosecutions should be required to prove the integrity of that data.

BCS said it wants an “end to the legal presumption that computer systems data is always correct, with no burden on the prosecution to prove it”. Earlier this month, the Post Office contacted 540 former subpostmasters, informing them that it might have wrongly prosecuted them using unreliable computer evidence.

Fletcher added: “The Horizon case uncovered a range of issues that are key to the reputation of our industry, including the relationship between technology and organizational culture, as well as the vital importance of meeting independent standards of professionalism, trust, and ethics.” Computer Weekly first revealed the scandal in 2009 with the stories of seven sub-post managers (see timeline below of Computer Weekly articles since 2009).

Sam De Silva, the partner at international law firm CMS and chair of the BCS’s Law Specialist Group, said the Post Office scandal highlights the dangers of accepting the output of automated systems without question. “It was for the subpostmasters to prove that the outputs and logs from the computer system were flawed or inaccurate,” said De Silva. “Yet how could non-IT specialists be expected to prove this when even some experienced IT professionals would find it challenging to do so?”

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