Downing Street was on Sunday night scrambling to protect Keir Starmer as it emerged No 10 was aware of the fatal allegation against Lord Mandelson days before he was sacked.
The Daily Mail understands that No 10 knew at the start of last week that the Labour grandee had suggested paedophile Jeffrey Epstein's first conviction was wrongful and should be challenged.
But sources insisted that the Prime Minister was not informed before he expressed confidence in the then-US ambassador in the Commons on Wednesday.
It came amid a deepening revolt over Sir Keir's handling of the crisis, and growing speculation that the PM could be ousted before the next general election.
Mutinous MPs yesterday warned that Sir Keir was 'supping in the last-chance saloon', while the Tories accused him of going 'missing in action' over the scandal.
The Prime Minister is expected to break his 'damning silence' on the crisis on Monday, but on Sunday night Downing Street would not reveal who kept the information about Lord Mandelson from him.
In the worst week of his premiership, Sir Keir sacked the Labour peer from his post as ambassador to the US.
It came after emails were published showing that Lord Mandelson had sent supportive messages to Epstein even as he faced jail for sex offences.
Sources insisted that the Prime Minister was not informed before he expressed confidence in the then-US ambassador in the Commons on Wednesday.
But it has emerged that Downing Street became aware of the damning correspondence on Tuesday - two days before he was sacked.
The Daily Mail can reveal No 10 was told then that the emails contained suggestions by Lord Mandelson that Epstein's first conviction was wrongful and should be challenged.
But sources insisted that the information, which came in a 2,000-word memo from Bloomberg News which was seeking comment from Lord Mandelson, was not shared with Sir Keir, who told MPs during Wednesday’s Prime Minister’s Questions that he had ‘confidence’ in his man in Washington.
Meanwhile, Foreign Office permanent secretary Sir Oliver Robbins, formerly Theresa May’s bungling Brexit chief, was seeking a response from Lord Mandelson about the emails, which only came later – on Wednesday afternoon.
On Thursday morning, the Foreign Office said that the ‘suggestion that Jeffrey Epstein’s first conviction was wrongful and should be challenged is new information’ and that ‘in light of that... he has been withdrawn as ambassador with immediate effect’.
The revelation adds to pressure on the PM’s all-powerful chief of staff, Morgan McSweeney, with several MPs calling for him to be sacked. Questions are being asked whether it was Mr McSweeney who knew in Downing Street, and if not, who?
The Mail on Sunday reported that Sir Keir and his top adviser had been involved in a spat over what the PM was or wasn't told following his Commons appearance last Wednesday, with the Labour leader roaring: 'You are supposed to protect me from things like this!' Downing Street denied the row.
Tory leader Kemi Badenoch said on Sunday night: 'The Prime Minister has gone missing in action. Serious questions have been raised about what he knew, and when, regarding Lord Mandelson's links to Jeffrey Epstein.'
It comes after emails were published showing that Lord Mandelson had sent supportive messages to Epstein even as he faced jail for sex offences.
Lord Mandelson with Jeffrey Epstein on December 12, 2005, where he is seen trying on a belt during a visit to a boutique in the Caribbean.
A Cabinet minister said Lord Mandelson’s ‘singular talents’ meant he was deemed ‘worth the risk’ to appoint as ambassador.
Peter Kyle, the Business Secretary and an ally of Lord Mandelson’s, suggested security checks carried out before his appointment only ‘turned up information that was already public’.
‘Now of course we have seen the emails which were not published at the time, were not public and not even known about, and that has changed the situation,’ he said.
One Labour MP said: ‘Nobody in the parliamentary Labour Party is happy with the handling... it will be incredibly difficult for the Prime Minister to survive in the long term.’
Labour's Graham Stringer said Sir Keir was 'supping in the last-chance saloon'.
He told Times Radio that the PM was 'vulnerable' and it was 'a given' among MPs that he was 'doing poorly at the job'.
Fellow Labour MP Rosena Allin-Khan said: 'The polls are going to show in May whether or not people believe [Sir Keir] is up to the job.'