The former chair of the Post Office has expressed “sincere regret” for the state-owned body’s failings in the Horizon IT scandal and said it was a mistake not to show a key report on the problem to its board.

Tim Parker, who was chair of the Post Office between 2015 and 2022, told a judge-led public inquiry that he felt “deep sympathy” for the Post Office operatives affected by what MPs have described as one of the biggest miscarriages of justice in UK history.

The Post Office wrongly prosecuted hundreds of operators because of financial shortfalls recorded in their branches but it has since emerged that the Horizon IT computer system was unreliable and suffered bugs, errors and defects.

Parker told the inquiry that there were “failures at all levels” of the Post Office over the way it defended the high court lawsuit brought against it by Sir Alan Bates and 554 other Post Office operatives, and it “may have relied too heavily on the advice of lawyers in the way the case was conducted”.

Bates’ group gained a significant victory in 2019 when a judge ruled in its favour, finding issues with the IT system.

Parker – a former private equity veteran whom unions once dubbed “the Prince of Darkness” because of his cost-cutting tactics – said that when he took over as chair the business was in “deep crisis”. He said it “absorbed billions of pounds of taxpayer money and was still losing money”.

After he took over, he commissioned a report from barrister Jonathan Swift looking at the Post Office’s handling of complaints made by operatives about the Horizon system after government ministers raised concerns.

Parker told the inquiry that, after Swift completed his report in January 2016, it was not given to the Post Office board because he had been told by Jane MacLeod, then the Post Office’s general counsel, that the Swift report was confidential and covered by legal privilege.

“I felt erroneously that legal privilege meant that the report effectively was circumscribed,” he said.

Ministers were also briefed on the report but were not given a hard copy amid fears the document could become publicly disclosable through freedom of information requests, the inquiry heard.

The Department for Business, Energy and Industrial Strategy later wrote to Parker in October 2020 telling him it had been a “mistake” not to have shown the Swift report to the board.

“At the time I was advised and I took that advice,” Parker told the inquiry, adding that, in 2016, he believed he was prevented from sharing the report with other board members.

He accepted that if the Swift report had been discussed at board level it could have led to a “different approach” by the Post Office to the high court lawsuit.

“Could we have shared it? I wish we had in a way,” he said. “What possible motive would I have at the time from hiding this report from my fellow board members other than receiving advice that I shouldn’t share it?… I had no vested interest in trying to protect the Post Office. It was simply the advice I received and I followed it.”

He added: “I would accept with hindsight [this] was a misjudgment. I am not a lawyer and I received very strong legal advice which I took … I did that in good faith. I had no reason to deliberately hide this thing or chuck it into the long grass.”

He said the board had been briefed by former chief executive Paula Vennells about recommendations from the Swift report and the government “certainly knew of the report”.

Parker is a high-profile figure in the business world and is the former chief executive of shoemaker Clarks, Kwik-Fit, the AA and luggage firm Samsonite, and a former chair of National Trust.

The inquiry heard that, when Parker took over as chair in 2015, he was working a minimum of 1.5 days a week but by November 2017 he had asked for that to be reduced to two days a month.

Parker told the hearing he would “certainly rebut” any suggestion that he had not spent enough time at the Post Office. “I would say I was an active, energetic chair who took a lot of time and spent time with people to understand the business,” he said.

The inquiry continues.