It was Jon Longman's job for 12 years from 2000 to uncover crime committed by employees, including sub-postmasters, and it has since emerged that security advisers at the company were incentivised to seek prosecutions through bonuses for convictions.

Mr Longman, who gave evidence to the scandal inquiry remotely for medical reasons, said media articles made him aware of a growing number of complaints about the Horizon system, operated by Fujitsu.

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He was involved in a number of high profile cases, including that of West Byfleet sub-postmistress Seema Misra who was jailed for 15 months while pregnant in 2010 - a conviction that was overturned by the Court of Appeal in 2021.

Mr Longman said he agreed that a proven issue with the Horizon system in a previous case should have been disclosed to her, assisting the work of her defence team.

He added that he was not a barrier to disclosures that may have prevented her ordeal, saying he would have been happy to hand over any information but it was ultimately not often his decision to make.

The inquiry heard that Post Office lawyers warned her defence team that the retrieval of transaction data relating to her case was "not a free service".

Commenting more widely on his investigative work, Mr Longman's witness statement added: "At the time, I do not believe that I considered a challenge to the Horizon system in one case to be relevant to other cases.

"As explained earlier in this statement, it was never confirmed to me that there was a definite fault with the Horizon system. As such, I was under the impression that the system was operating as expected."

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Mr Longman admitted suspicions of a fault with Horizon in the case of Jennifer O'Dell, who was wrongly accused of stealing more than £9,600.

He added that cost was a factor in the decision not to request transaction data from Fujitsu in her case, placing a greater burden on her defence team to fight the theft charges.

Limits were in place for the number of audit record queries (ARQs) that could be made each year, the inquiry heard, sometimes taking up to three years for data from requests to be made available.

Mr Longman conceded he did not consider disclosing evidence of Horizon problems in Mrs O'Dell's case to Ms Misra's legal team.