A blame-game is under way following the ITV drama Mr Bates vs The Post Office, which depicted how hundreds of sub-postmasters and sub-postmistresses were wrongly held responsible for accounting errors created by the faulty Horizon IT software.

Sir Ed Davey and Lord Peter Mandelson, who are both facing questions for their roles as postal affairs minister and business secretary during the scandal, have laid some of the blame at the door of civil servants - with the latter arguing that officials should have been "more focused and cognisant of what was going on" and that they "failed" to protect ministers.

And in an interview with Sky News, Sir Ed, the Liberal Democrat leader, accused officials in the Department for Business, Innovation and Skills of "lying to me" over the scandal.

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Mark Serwotka, general secretary of the Public and Commercial Services Union, told Sky News that Lord Mandelson's comments were "just another cowardly example of politicians scrambling to blame others for their own mistakes".

"Politicians are quick to take credit when things go well. They should be humble and honest enough to take responsibility for their mistakes, and swiftly deliver justice for the wronged sub-postmasters and sub-mistresses."

Dave Penman, the general secretary of the FDA union for civil servants, branded Sir Ed's comments "outrageous" and said they were an "act of desperation from a former minister trying to save his own skin".

"Ed Davey goes beyond what is reasonable to expect from a former minister," he told Sky News. "If he's going to repeat this he needs to back up his accusations."

He added: "Civil servants should rightly be held to account for what they did and didn't do, but they need an opportunity to defend themselves."

Between 1999 and 2015, more than 700 people were prosecuted for a variety of offences including theft, fraud and false accounting - causing many to lose their jobs, livelihoods and reputations.

On Wednesday, Rishi Sunak announced that a new law would be introduced to exonerate and compensate those caught up in the Horizon scandal and that those who were part of the group litigation order against the Post Office would also be eligible for an upfront payment of £75,000.

Sir Ed - who was postal affairs minister between 2010 and 2012 during the coalition government - has attracted particular criticism after it emerged he refused to meet sub-postmaster Alan Bates, whom the ITV drama is named after, on a number of occasions - saying in a short three-paragraph letter that a meeting "wouldn't serve any purpose".

However, Sir Ed did later meet with Mr Bates and was the first minister on public record to do so. It is understood he then asked his officials to follow up on the concerns raised by the sub-postmaster at their meeting.

Speaking to Sky News this week, Sir Ed said: "I wish I'd known then what we all know now. The Post Office was lying on an industrial scale to me and other ministers.

"When I met Alan Bates and listened to his concerns, I put those concerns to officials in my department, to the Post Office and to the National Federation of Postmasters and it's clear they all were lying to me."

Sir Ed was joined in his criticism of the civil service by Lord Mandelson, who was in charge of the oversight the Post Office from 2008 until his departure in 2010.

In his Times Radio podcast aired earlier this week, Lord Mandelson said: "I'm not trying to point the finger at particular civil servants obviously," he added, "but they should have been much more focused and cognisant of what was going on.

"And their job is to, in a sense, both to protect ministers and serve the wider public interest, and in this instance that failed."

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Separately, it has emerged that Lord Mandelson was made personally aware of the Horizon issue, after an email released under the Freedom of Information (FOI) showed they were highlighted to him in 2009 by Tory peer Lord Arbuthnot.

However, rather than respond himself, the FOI instead showed a response from his junior business minister, and now close ally of Sir Keir Starmer, Pat McFadden, who said there was "nothing to indicate that there are any problems with the Horizon system".

A spokesperson for the Liberal Democrats said: "Ed has been clear that Post Office managers lied to the victims, to judges and to ministers, those lies circulated across the entire system.

"It is also the case that like all former ministers and post office managers, civil servants - particularly those sitting on the post office board, must face the inquiry and answer questions.

"The priority now needs to be getting justice and compensation for the victims."

The Labour Party has been approached for comment.