Lord Mandelson helped his "best pal" Jeffrey Epstein through "years of torture" over teenage sex allegations, leaked emails suggest.
While the paedophile was facing charges in June 2008, Peter Mandelson wrote: "Your friends stay with you and love you."
The leak comes hours after the British ambassador to the US warned that embarrassing correspondence with his former friend was set to emerge.
Lord Mandelson issued an apology for remaining friends with Epstein after his sex convictions.
Sir Keir Starmer gave his full backing to Lord Mandelson at PMQs today, saying: "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."
Sir Keir insisted that "due process was followed" during the appointment of the ambassador.
A series of messages has been leaked from Lord Mandelson, where he wrote to Epstein: "I think the world of you, and I feel hopeless and furious about what has happened.
"I can still barely understand it. It just could not happen in Britain. You have to be incredibly resilient, fight for early release and be philosophical about it as much as you can."
LATEST DEVELOPMENTS
Peter Mandelson told Epstein 'to be incredibly resilient'
|GETTY
The emails, sent between the pair's private emails and leaked to The Sun, continued: "Everything can be turned into an opportunity and that you will come through it and be stronger for it.
"The whole thing has been years of torture, and now you have to show the world how big a person you are, and how strong."
Another email showed Mr Mandelson quoting ancient Chinese general Sun Tzu, reading: "Reminder. You are fighting back, so you need strategy, strategy, strategy. Remember the Art of War."
Lord Mandelson has not disputed the veracity of the messages, but emphasised his apology first given in an interview to Harry Cole.
Epstein and Lord Mandelson exchanged messages on their private emails
| Getty ImagesHe admitted to remaining close to Epstein "for far longer than I should have done".
The ambassador pleaded his "deep regret" at remaining in contact for "many years" with the predator.
Speaking to Harry Cole, Mr Mandelson said: "I feel a tremendous sense, a profound sense of sympathy for those people, those women, who suffer as a result of his behaviour and his illegal criminal activities.
"And secondly, I regret very, very deeply indeed, carrying on that association with him for far longer than I should have done.
Kemi Badenoch called for Lord Mandelson to be sacked
| PARLIAMENT.TV"I regret very much that I fell for his lies, I fell for and accepted assurances that he had given me about his indictment, his original criminal case in Florida, like very many people, I took at face value what he said with hindsight, with fresh information, many years later, we realised that we had been wrong to believe in is a charismatic criminal liar."
Multiple MPs have called for Lord Mandelson to be sacked, including Sir Ed Davey and Tory leader Kemi Badenoch.
Mrs Badenoch said: "These are sickening revelations. Mandelson's position is untenable. Why did Starmer defend him today? How was 'full due process followed'?
"This is a weak Prime Minister, leading a Government mired in scandal. The public deserves better. Peter Mandelson needs to be fired now."