Sir Keir Starmer has left the door open for Lord Mandelson to stay on as a peer after his close ties to Jeffrey Epstein were revealed.
The former ambassador to the United States was sacked from his role after it emerged last month that he had sent emails to convicted sex offender Epstein before he was sentenced in 2008.
Lord Mandelson was reported to have told Epstein to "fight for early release" shortly before he was sentenced to 18 months in prison and said "I think the world of you," the day before the disgraced financier began his sentence for soliciting prostitution from a minor in June 2008.
He also described Epstein as "my best pal" in a message for his 50th birthday in 2003.
Speaking to reporters during his trade mission in Mumbai, the Prime Minister was asked if he could see a future for Lord Mandelson in frontline politics.
Sir Keir replied: "Not in a Government role in terms of future appointments.
"I think Peter is also on a leave of absence from the Lords in any event so the issue of the whip doesn’t arise."
Lord Mandelson technically does not have the Labour whip in the Lords, as when he started his post as ambassador to Washington, he took a leave of absence from Parliament’s upper chamber.
Sir Keir has ruled out a future for Lord Mandelson in frontline politics
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He has previously said he continued his association with Epstein, who died in a New York jail cell in 2019, "for far longer than I should have done".
Sir Keir last month said he would "have never appointed" Lord Mandelson if he had known about his ties to Epstein.
Asked why he had been appointed, the Prime Minister explained: “Let me talk this through with you, because it’s important to appreciate what happened when. And I understand that obviously the post, the diplomatic post of American ambassador [is a] really important post.
“And Peter Mandelson, before he was appointed, went through a due diligence process. That’s the propriety and ethics team. He went through a process, and therefore I knew of his association with Epstein.
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Lord Mandelson has express 'regret' over remaining in contact with Epstein
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"But had I known then what I know now, I’d have never appointed him, because what emerged last week were emails, Bloomberg emails, which showed that the nature and extent of the relationship that Peter Mandelson had with Epstein was far different to what I had understood to be the position when I appointed him."
Lord Mandelson expressed his "deep regret" at remaining in contact with the paedophile financier following the emergence of the leaked emails.
He told journalist Harry Cole: "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."
The former ambassador was stripped of his university honours by Manchester Metropolitan University last month, which said it "no longer has any ties" with him.
It was announced last week that he will also be stripped of the Freedom of Hartlepool, where he was a Labour MP.