PRESSURE mounted on Sir Keir Starmer to resign today as backbenchers joined opposition parties saying the PM must have been aware Lord Peter Mandelson had failed vetting before appointing him US ambassador.

The Labour leader said it was “unforgivable” that he was not told that the Foreign Office had overruled the UK Security Vetting team’s recommendation not to clear the so-called “Prince of Darkness” for the post.

“That I wasn’t told that Peter Mandelson had failed security vetting when he was appointed is staggering,” the PM said.

“That I wasn’t told that he had failed security vetting when I was telling Parliament that due process had been followed is unforgivable.

“Not only was I not told, no minister was told, and I’m absolutely furious about that.”

Sir Keir will face MPs on Monday under pressure from opponents to resign for misleading Parliament about the situation after the Foreign Office’s top civil servant, Sir Olly Robbins, was sacked on Thursday night over the scandal.

He insisted he would “set out all of the relevant facts” to MPs to offer “full transparency and full accountability.”

But veteran Labour MP Diane Abbott MP told Sky News today: “It’s just not possible that No 10 didn’t know.

“And I think to try and pretend No 10 didn’t know, when anybody who follows those issues would know that that can’t be true, is the kind of thing that undermines people’s faith in politics.”

She added: “It’s always been the process that if you mislead the house you have to step down and I do think the Prime Minister must consider his position.”

With Labour expecting a drubbing in the May elections, fellow backbencher Jon Trickett MP added: “It simply doesn’t sound credible for Keir Starmer to claim that he was unaware that Mandelson had been denied security clearance. 

“If the PM did not know, it raises gravely serious issues about the way we are governed. 

“Either way, the excuses coming from Downing Street won’t cut it on the doorstep in the run-up to the local elections.”

Maryam Eslamdoust, general secretary of the Labour-affiliated TSSA union, called for a Labour leadership contest, warning: “Labour is in danger of being irreversibly tainted by this latest instalment in the Mandelson scandal and Keir Starmer’s handling of it.”

She added: “Keir Starmer’s claim that he knew nothing about Mandelson’s failed security clearance is a very hard sell to voters.

“It’s clearly not good enough, and at the very least, Keir has lost control by presiding over such reckless conduct.”

Lord Mandelson, a political appointment rather than a career diplomat, was sacked from his Washington role last September when more details emerged about his relationship with paedophile financier Jeffrey Epstein, who died in 2019.

Sir Keir was already under fire over the decision to give the twice-resigned cabinet minister the job, despite it being known that his dealings with Epstein continued after the financier’s conviction for child sex offences.

Questions over his judgement intensified after the first batch of documents related to the decision, published last month, showed that he was warned before announcing Lord Mandelson’s ambassadorship of a “general reputational risk” over his association with Epstein.

That warning stemmed from the first part of the checks, carried out by the Cabinet Office, which was based on information in the public domain at the time.

Communist Party general secretary Alex Gordon said: “Keir Starmer’s political career is over. He is unable to lie straight in bed.

“Having already lost a director of communications and now the senior Foreign Office mandarin over his appointment of Peter Mandelson as ambassador to Washington, the question of how Starmer’s ‘forensic’ legal mind could have missed what the dogs in the street knew, that Mandelson was a security risk, will keep on coming and more evidence of Starmer’s lies could yet come out. 

“If backbench Labour MPs had a 1922 Committee it would surely be meeting now. As it is, Starmer’s appearance before the House of Commons on Monday must surely be his last as PM.”

The Public and Commercial Services union called today for an immediate halt to the Foreign Office restructuring programme following the departure of Sir Olly.

Chief Secretary to the Prime Minister Darren Jones told broadcasters today that Sir Keir had not lied to the Commons and was not considering his position as a result of the furore.

Conservative leader Kemi Badenoch said there had been “deliberate dishonesty” while the Liberal Democrats called for both Parliament’s privileges committee — the same body that investigated Boris Johnson over the Covid-19 partygate affair — and Sir Keir’s ethics adviser Sir Laurie Magnus to consider whether the PM misled the house.

The SNP, Green Party and Reform UK also called for Sir Keir to resign.

Scottish Labour leader Anas Sarwar, who called for Sir Keir to quit in February, said the Mandelson affair had been “the tipping point for me” and “I stand by my position.”

Documents produced by UK Security Vetting are tightly controlled.

Mr Jones, the senior Cabinet Office minister, said that he had suspended the right for the Foreign Office and a “small number” of other organisations to overrule recommendations from the specialist unit.

“They go through financial, personal, sexual, religious and other types of background information and that is why it is kept extremely private on a portal that only a few people have access to,” he said.

“The Prime Minister was not given those documents because he would not routinely be given them about individuals’ appointments.”