A former chair of the Post Office has described the business as “very badly run and messy” and criticised the Horizon IT system as clunky, creaky and badly in need of replacing.

Neil McCausland, a senior independent director and interim chair of the Post Office between 2011 and 2016, heavily criticised the state of the business and its IT set-up at the public inquiry into the Horizon scandal on Monday.

“The Post Office I went into was a very badly run and messy business,” said McCausland, a former executive at Marks & Spencer and C&A. “The IT infrastructure we knew was old, underinvested and creaky. The Horizon system we knew was near end of life.”

McCausland, whose role was in part looking at how to put in place a new system to replace the Fujitsu-developed Horizon accounting software in the future, said it was at least 15 years old by then, when usual practice is to replace IT systems after about a decade.

“We knew we had clanky, underinvested IT infrastructure,” he said. “Horizon was a clunky, creaky, not particularly intuitive system.”

However, McCausland said he was given no indication that the integrity of the data and the accounting software was anything other than “sound”.

In his first board meeting where Horizon’s integrity was discussed, before the Post Office was separated from Royal Mail in 2013, the system was given a clean bill of health.

He said he was told the legal claims about its integrity were “weak” and that audits done on Horizon were “positive”.

“[I was told] the all previous prosecutions where Horizon data was used we had been successful,” he said. “We were listening to an experienced Royal Mail Group director who was talking to our general counsel. It was a very clear and robust answer … that the system was sound. Data integrity wasn’t an issue in my mind.”

McCausland was also critical of Paula Vennells, who was chief executive of the Post Office from 2012-19, suggesting she kept the board in the dark by offering limited information and poor briefings on investigations into the Horizon system, including a key report by the forensic accountants Second Sight.

He told the inquiry that Vennells had shoehorned into a board phone call under “other business” the fact the Second Sight findings were about to be published.

The report, which ultimately became crucial for the branch owner-operators’ campaign to clear their names, looked at 47 cases to try to see if the losses at branches could have been due to technical IT issues, and found that two bugs had affected 76 branches.

“I was upset that we were being told a week before the [interim] report was going to come out,” he told the hearing. “To have it dropped on us not in proper written communication, not at a proper board meeting and without proper papers.

“It was thrown in as ‘any other business’ and Vennells said, ‘We are not happy with it, it is inaccurate and by the way there are these bugs and flaws.’ I was not happy about it, and the other non-executive directors were not happy about it. The way it was launched on the board was unacceptable.”

Between 1999 and 2015, the Post Office relentlessly pursued hundreds of branch owner-operators across the UK for alleged theft, fraud and false accounting, despite knowing there were faults in Horizon software installed by Fujitsu in the late 1990s.

In total, about 3,500 post office operators were wrongly accused of taking money from their businesses, and more than 900 were prosecuted in harrowing court cases despite protesting their innocence and raising issues with the IT system in their defence.

The scandal is frequently described as the most widespread miscarriage of justice in UK history.