One of the architects of the Post Office Horizon accounting system has admitted there were “discreet” bugs but it generally “worked well”, dismissing suggestions he had knowledge of widespread flaws.

Gareth Jenkins, who was a lead engineer at Horizon supplier Fujitsu, told the public inquiry into the IT scandal that while pilots for both Horizon systems encountered troubles, systemic issues he was aware of were ironed out.

Under questioning from counsel to the inquiry Jason Beer KC, he believed the scale and seriousness of the bugs in the system that have been complained about was inconsistent with his own understanding of Horizon’s integrity.

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He said that applied to both the original software, now known as ‘legacy Horizon’, and the later Horizon Online system which, he pointed out, was still being used to this day.

Mr Jenkins was in the paid employment of Fujitsu from 1996 to 2022.

He was utilised, by the Post Office, as an expert witness in prosecutions of sub-postmasters on charges such as theft and false accounting.

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He gave evidence in the pivotal 2010 trial of Seema Misra, who was jailed while pregnant.

It has been alleged that Mr Jenkins failed to disclose then the existence of a known bug in the accounting system that had the potential to clear her name and halt other prosecutions.

In addition to knowledge of flaws in Horizon, he is facing further claims relating to the ability of Fujitsu personnel to access the legacy system without the knowledge of sub-postmasters.

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He is being investigated by police on perjury grounds.

The law states that expert witnesses in criminal cases must be impartial.

In his witness statement to the inquiry, Mr Jenkins said he was “truly sorry” she was wrongly convicted, adding that it was “even more tragic” that she was pregnant at the time – a fact, he said, he did not learn until some years later.

The inquiry has already heard Mr Jenkins was used on multiple occasions to provide information as the Post Office took sub-postmasters to court.

“The idea that I would lie about Horizon, knowing that an innocent person could be convicted and imprisoned, is completely abhorrent to me”, he wrote.

He said on Tuesday that the duties of being an expert witness were not explained to him.

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Mr Jenkins told counsel he did not understand his duties of disclosure until 2020.

Mr Jenkins denied having knowledge of bugs other than those he was personally asked to investigate.

“I’m not sure that even today I understand what bugs actually did cause the problems that people suffer from,” he added, saying that those he was alerted to were “discreet” and “well controlled and managed at the time”.

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Asked if it had occurred to him that he should have made sure he knew about the problems that had not been referred to him before he went to court, Mr Jenkins replied: “That didn’t occur to me.

“I was confident, possibly wrongly so, that if problems did occur they were quickly fixed and not left to fester in the system to have a large impact.

“With hindsight I would have done things differently,” he added, saying he would have asked wider questions.

More than 700 sub-postmasters were wrongly convicted of crimes linked to their use of the Horizon systems.

Mr Jenkins is scheduled to give four days of evidence.