By Tom Espiner, BBC business reporter
BBCFormer Fujitsu engineer and Horizon expert Gareth Jenkins has said he didn’t think the software was “a monster” and that he was “trapped into doing things I shouldn’t have done”.
Speaking at an inquiry into the Horizon scandal, Mr Jenkins was accused by lawyer Flora Page of “protecting” it when he gave evidence as an expert witness between 2010 and 2013.
Ms Page represents Horizon victims including former sub-postmaster Seema Misra, who Mr Jenkins gave evidence against in 2010.
Ms Misra told the BBC the system was a monster which “had blood on its hands, and so do the people who protect [it]”.
Between 1999 and 2015, the Post Office prosecuted hundreds of sub-postmasters on the strength of faulty Horizon data, with 700 convicted of offences such as theft and false accounting.
Horizon falsely flagged discrepancies in accounts, with many sub-postmasters using their own savings to make good the imaginary shortfalls.
The wrongful prosecutions, which have been called one of the biggest British miscarriages of justice, had a devastating effect on the lives of the sub-postmasters affected.
They faced financial ruin and the loss of standing in their communities.
Some sub-postmasters ended up taking their own lives after being pursued by the Post Office for supposed losses, including Martin Griffiths, whose widow was later paid in instalments by the Post Office for her silence, the inquiry heard in April.
Giving his fourth day of evidence at the inquiry into the Horizon scandal, Mr Jenkins was questioned by Ms Page, a barrister who represented Horizon victims including Seema Misra.
Ms Misra was pregnant with her second child when she was convicted of theft and sent to jail in 2010. She later told the BBC she would have killed herself had she not been pregnant at the time.
In a charged atmosphere at the hearing, with Ms Misra sitting next to her, Ms Page accused Mr Jenkins of being a “Fujitsu man”.
She said Mr Jenkins “knew the Misra trial was a test case for Horizon”.
Ms Page alleged his role was to make sure Horizon had a “clean bill of health” and that he had “tailored” his evidence accordingly.
“My role was to tell the truth,” Mr Jenkins said. “I attempted to answer as best I could the questions I was asked”.
Ms Page responded: “Never mind whether a by-product of protecting the monster was that a woman was sent to jail”.
Mr Jenkins said he was “sorry for had happened to Mrs Misra” but he felt that was down the to way that the Post Office had behaved and wasn’t “purely down to me”.
“I clearly got trapped into doing things I shouldn’t have done, but that was not intentional on my behalf,” he said.
Mr Jenkins was part of the Fujitsu team that developed the Horizon software, and part of his role was dealing with bugs in the system.
Mr Jenkins has maintained throughout his evidence that he didn’t understand his previous duties as an expert witness to tell the court about problems with Horizon, and that he didn’t know that he shouldn’t just narrowly answer questions about the technology.
Under questioning at the inquiry on Friday, Mr Jenkins said: “I appreciate that I got things very wrong, but it was done through ignorance rather than maliciousness”.
He added that the Horizon system “as a whole was working well”.
Mr Jenkins is currently being investigated by the Metropolitan Police on suspicion of perjury – lying to a court, and perverting the course of justice. No criminal charges have been brought.
Responding to Mr Jenkins’ questioning, Ms Misra told the BBC that Horizon is a “monster who killed people”.
She said she didn’t believe Mr Jenkins’ testimony, and that he is “just trying to cover his own back”.
Ms Misra added that on a personal level, hearing Mr Jenkins’ testimony over the past days had been “really bad”.
“I had to go into the loo to cry and scream,” she said. “I didn’t want to show my tears to him”.
However, getting the testimony out in the open had been “good”, she added.
“That gives me the power to keep on fighting,” she said, and that she felt she has the UK “nation behind me”.