Sir Keir Starmer has been left scrambling to defend Peter Mandelson as the Conservatives demand all the messages with paedophile Jeffrey Epstein be published.
Tory leader Kemi Badenoch asked Sir Keir at Prime Minister's Questions: "Does the Prime Minister have full confidence in Peter Mandelson?"
The Prime Minister responded: "Let me start by saying the victims of Epstein are at the forefront of our minds. He was a despicable criminal who committed the most heinous crimes and destroyed the lives of so many women and girls.
"The ambassador has repeatedly expressed his deep regret for his association with him; he's right to do so.
"I have confidence in him, and he's playing an important role in the UK-US relationship."
Mrs Badenoch responded: "The Prime Minister cannot answer any questions. That is not the behaviour of someone who has full confidence."
Three Labour MPs have since broken ranks, calling for Lord Mandelson to be sacked.
Bell Riberio-Addy, Kim Johnson and an MP who commented anonymously all said he should be fired as the UK's ambassador to the US.
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Sir Keir Starmer has been left scrambling to defend Peter Mandelson (above)
| PAMs Ribbery-Addy said: "There should definitely be an investigation into it because there will be a huge amount of concern, and if the outcome is that he should resign."
The MP for Liverpool Riverside, Ms Johnson, told The Telegraph: "I totally agree [with Bell]. But the fact is, this was known before he was given this position in America as the ambassador.
"As much as he says on the radio that he regrets it, the fact is that he called him his 'best pal' in his birthday card, he made money from him.
"That suggests to me a very strong relationship because who sends cards with those comments on? He should never have got it in the first place."
The third MP echoed the pair's thoughts, stating: "Of course, he should go. And he has had good practice in resigning. He probably has a standard letter on his website."
Liberal Democrat leader Sir Ed Davey joined the trio, calling for a probe into Lord Mandelson.
Sir Ed said: "Lord Mandelson has serious questions to answer over his relationship with the vile sex trafficker and paedophile Jeffrey Epstein.
"The Civil Service Commission needs to look into whether the ambassador has broken the diplomatic service code by failing to come clean over this sooner. With Trump in the White House, we cannot afford to have an ambassador to the United States who is compromised by' embarrassing' secrets."
Sir Keir was left scrambling at Prime Minister's Questions
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Sir Keir refused to reveal what checks were carried out on Lord Mandelson over his friendship with Epstein.
Downing Street insisted that the UK ambassador to the US was thoroughly vetted amid the allegations.
A spokesman for Number 10 said: "We have been very clear that the victims of Jeffrey Epstein are at the forefront of our minds, he was a despicable criminal who committed the most heinous crimes and destroyed the lives of so many women and girls.
"On the appointment process, any candidates for ambassador positions are subject to extensive vetting and background checks by the Foreign Office and Cabinet Office as a matter of course."