The Post Office “has turned itself into the nation’s most untrustworthy brand” as a result of its attempts to “protect” its defective Horizon IT system from concerns about its reliability, the Court of Appeal has heard.

Dozens of former subpostmasters who say they were wrongly convicted of theft, fraud and false accounting as a result of the Horizon accounting scandal, many of whom were jailed, are battling to finally clear their names this week.

The Criminal Cases Review Commission (CCRC) referred the cases of 42 former subpostmasters to the Court of Appeal last year, following a landmark civil case against the Post Office.

A High Court judge found the Fujitsu-developed Horizon accounting system contained “bugs, errors and defects” and that there was a “material risk” shortfalls in branch accounts were caused by the system.

As a result of the High Court’s findings, the CCRC considers there is “a real possibility” that the 42 subpostmasters’ convictions are unsafe, and their appeals began at the Royal Courts of Justice in London on Monday.

Tim Moloney QC, opening the appellants’ case, said that “the lives of subpostmasters were irreparably ruined and some took their own lives”, but the Post Office still did not commission “an independent investigation of the reliability of Horizon”.

He said the Post Office’s failure to do so was “shameful and culpable”, adding: “Those failures are rendered all the more egregious … by the inability of the defendants to make their own investigations of the reasons for the apparent discrepancies.”

Mr Moloney continued that the appellants were “reliant for disclosure on a business that was the complainant as well as the prosecutor and which demonstrated an enduring institutional resistance to disclosure” of anything which “undermined the reliability of Horizon”.

He also said that “there were conditions imposed on plea deals”, requiring defendants to make “no criticism” of the Horizon system.

Mr Moloney told the court that there was “an institutional imperative of acquitting Horizon and convicted subpostmasters … in order to protect Horizon and to protect their own commercial reputation”.

He said many of the appellants had “pleaded guilty in the face of the difficulties that they faced in defending themselves, being deprived of any meaningful way of defending themselves”.

Mr Moloney added: “All had the shame and humiliation of arrest and prosecution.

“All experienced the enormous psychological toll associated with that process.”

He added that many “received a custodial sentence – many immediately went to prison”.

Mr Moloney continued: “Some saw their marriages break up, others suffered bankruptcy and some are dead, having gone to their graves with their previous convictions still extant.”

The court heard that there were “concerns” about the Horizon system “from the very outset”, and that “the very highest levels of management and governance in the Post Office were on notice of the real potential for Horizon to malfunction and misfire”.

But, Mr Moloney added, the Post Office “chose to disbelieve the subpostmasters … it chose to ignore the distress that was being suffered by those subpostmasters”.

Mr Moloney said: “Essentially, the Post Office, in the face of all the evidence, was prepared to accept that subpostmasters with previous good character, who had hitherto run decent, responsible, profitable businesses, became criminals overnight. Alarm bells should have rung.”

Sam Stein QC, who also represents some of the appellants, told the court: “The Post Office has turned itself into the nation’s most untrustworthy brand … through its own behaviour and its own fault over many years.”

He added that there was “a wealth of evidence” which showed the Post Office had “a general hostility towards subpostmasters, the people who are – for the rest of us – at the heart of the community”.

The Post Office has conceded 39 of the 42 appellants’ appeals should be allowed, on the basis that “they did not or could not have a fair trial”.

But it is contesting 35 of those 39 cases on a second ground of appeal, which is that the reliability of Horizon data was “essential to (their) prosecution and conviction” and their convictions were therefore “an affront to the public conscience”.

Four of the 42 appeals are not being opposed on either ground, while three are fully opposed by the Post Office, which has previously said it will not seek retrials of any of the appellants if their convictions are overturned.

In a statement, a Post Office spokeswoman said: “Post Office is not opposing the majority of these appeals and informed the court and appellants of this at the earliest opportunity, in October 2020.

“We sincerely apologise for historical failings and have taken determined action to address the past, ensure redress for those affected and prevent such events ever happening again.

“It would be inappropriate to comment further whilst there are continuing hearings at the Court of Appeal.”

The Post Office settled the civil claim brought by more than 550 claimants for £57.75 million, without admitting liability, in December 2019.

Mr Justice Fraser, who formally approved the settlement, also referred the case to the director of public prosecutions over “very grave concerns regarding veracity of evidence given by Fujitsu employees to other courts in previous proceedings”.

The hearing before Lord Justice Holroyde, Mr Justice Picken and Mrs Justice Farbey is expected to conclude on Thursday or Friday, and it is expected that they will give their ruling at a later date.