A former chair of Royal Mail has apologised for the “tragic and diabolically unfair” prosecutions of post office operators, but denied ever having been informed of suspected problems with the Post Office’s faulty Horizon computer system.

Allan Leighton, who chaired Royal Mail from 2002 to 2009, when it also owned the Post Office, told a statutory inquiry that had “absolutely not” received any warnings that the Horizon system was unreliable.

The Post Office is under intense scrutiny as an inquiry proceeds into the scandal over the Horizon computer system. Errors in the system, installed by the contractor Fujitsu, led to the wrongful prosecution of hundreds of post office operators for supposedly stealing money. The inquiry is scrutinising the failings that led to the scandal and whether it was covered up.

Rishi Sunak described the scandal as “one of the greatest miscarriages of justice in our nation’s history”, but the government promised to quash convictions based on Horizon evidence only in January. Post office operators campaigned for justice for years, but it was only after an ITV series dramatised the struggle of Alan Bates against his prosecution that the government took action.

Leighton is a former chief executive of supermarket Asda and jewellery brand Pandora, as well as chair of the board of a host of prominent companies ranging from the now defunct department store BHS to travel booker Lastminute.com and Selfridges. He was in charge of the Royal Mail board at the time that many of the wrongful prosecutions were carried out, but in evidence and a witness statement to the inquiry on Wednesday, he repeatedly said that he was not aware of any indications of problems with Horizon – including a letter from Bates.

He blamed the scandal on the failure of unnamed people within the Post Office to pass details of Horizon failings up to superiors, and said processes were not followed correctly. “It’s because of people that this happened,” he told the inquiry.

“What’s happened has been a terrible thing for everybody who has been involved in it, particularly the subpostmasters, subpostmistresses,” Leighton said. “It’s unbelievable that it’s happened and I just wanted to say that I’m sorry that the elements of that that occurred in my tenure at the Royal Mail, I am sorry for that happening.”

Leighton said that the Post Office board would not usually be expected to attend to detailed technical problems that affected many of the post office operators who tried to use the Horizon system, or the audits of the accounts of individual branches.