The Post Office’s Horizon system is “very robust” and the only scandal is that the company failed to properly defend it, according to a man tasked with representing sub-postmasters.

George Thomson, the former leader of the National Federation of Sub-Postmasters (NFSP), told a long-running inquiry: “The Horizon system is not the scandal, the Post Office stupidity on steroids handling of the situation is the scandal.”

Between 1999 and 2015, more than 900 sub-postmasters were prosecuted for offences including theft on the strength of faulty data from the Horizon IT system.

Christopher Head, a former sub-postmaster, described Mr Thomson’s views as “nothing short of abhorrent”.

David Enright, a lawyer who has represented hundreds of people in connection with the Post Office, said Mr Thomson’s “flat earther defence of the Horizon computer system defies credulity”.

Mr Thomson said that the number of sub-postmasters who had been prosecuted was a “tiny percentage” of the 100,000 people who have used Horizon over 25 years.

Horizon is a software system for tasks such as accounting and stocktaking which began to be installed across Post Office branches from 1999.

Sub-postmasters complained about bugs after it falsely reported shortfalls – often for many thousands of pounds – but their concerns were dismissed.

Mr Thomson said the Post Office’s “mishandling of the situation” has been “so catastrophic” for sub-postmasters, the brand and Royal Mail group.

It means that the Post Office has “not been able to defend a robust Horizon computer system”, Mr Thomson said.

Mr Thomson denied, however, the association that it became “too close” to the Post Office or was “flush with cash”.

The inquiry also heard the former union general secretary privately tipped off the Post Office about those “sniffing around” the Horizon IT system.

In an email from Mark Davies, communications and corporate affairs director at the Post Office, sent on May 10 2015, part of the chain read: “Our media relationships with George [Thomson] and team are very good at present – he has been tipping us off, privately, about people sniffing around Horizon.”

Asked by inquiry counsel Mr Blake if he was “tipping off” the Post Office, Mr Thomson said: “I have made the point time after time today that Horizon is a robust system”, adding that he worked closely with the Post Office to strengthen the franchise.

Mr Head said: “I think his evidence so far shows him for the man that he is. He has shown no remorse, no sympathy, there is absolutely no sign of any apology for his or the NFSP’s part in the scandal and the damage done in this scandal.

“The testimony so far is nothing short of abhorrent.”

The NFSP was a trade union – set up in 1897 by a group of sub-postmasters to help their profession – which was changed into a trade association a decade ago after the Post Office did not recognise the group for collective bargaining purposes.

During questioning at the inquiry on Friday, Mr Thomson’s responses were greeted by astonishment and anger by some sub-postmasters in the room.

When asked by Julian Blake, counsel for the inquiry, why the NFSP had not defended sub-postmasters, Mr Thomson insisted that the federation had argued on their behalf but had not had the funds for a legal defence.

The NFSP has received millions of pounds in payments from the Post Office.

The inquiry was shown an email from August 2013 that outlined plans for a 15- year deal between the Post Office and NFSP.

It included annual payments starting at £500,000 in 2013-14 and reaching £2.5m between 2017 and 2028.

Mr Thomson was general secretary of the NFSP between 2007 and 2018.

He said: “We worked closely with the Post Office because we both needed to have a successful franchise – that’s the reality.”

Following Mr Thomson’s evidence to the inquiry, Calum Greenhow, chief executive of the NFSP sought to distance the organisation from his comments, saying the union was “shocked”.

“We want to apologise unreservedly to any member who sought help from the NFSP about Horizon and didn’t receive the support they so desperately needed and deserved,” he added.

“It is clear that more could and should have been done for them and for others.”

Mr Greenhow said the evidence heard on Friday “showed” that during Mr Thomson’s tenure “he did not take seriously enough the significant warning signals about Horizon and was not willing to accept challenges to his views on it”.

The Horizon scandal was thrown back into the spotlight by an ITV drama broadcast at the beginning of the year: Mr Bates vs the Post Office.

It followed the story of Alan Bates, a former sub-postmaster and campaigner who took the Post Office to court, and a number of sub-postmasters who were wrongly accused of stealing money or falsifying records.

He has led a 20-year campaign for justice for sub-postmasters, and was knighted this month.

Mr Thomson criticised Sir Alan and the media for “painting Horizon as not fit for purpose”.

“This viewpoint is not only factually incorrect but has damaged the brand and post offices all over the UK,” he said.

“My support for Horizon has never wavered.”

He said although bugs had been found in the software, that did not mean Horizon was not systemically robust.

He added that he was “furious” with the Post Office for taking people to court when they knew “people could access your computer”, adding that the “shenanigans” were “bizarre”.