Learning the details of Peter Mandelson’s relationship with paedophile Jeffrey Epstein was “like a knife through my soul”, the Prime Minister’s former chief of staff has said.
Morgan McSweeney on Tuesday took responsibility for appointing the peer US ambassador as he apologised for his role in the scandal, saying Sir Keir Starmer had “relied on my advice and I got it wrong”.
He insisted he only learned the full extent of the friendship Mandelson shared with the sex offender financier through the release of the Epstein files by US authorities.
Earlier, former Foreign Office chief Sir Philip Barton told MPs that the Lord’s links to Epstein were a known “toxic hot potato” before he was handed the ambassador role and that Downing Street was “uninterested” in his security vetting.
“The nature of the relationship that I understood he had with Epstein was not a close friendship,” Mr McSweeney told the Foreign Affairs Committee.
“How I understood it at the time was a passing acquaintance that he regretted having and that he apologised for.
“What has emerged since then was way, way, way worse than I had expected at the time, and it was when I saw the pictures, when I saw the Bloomberg questions in September 2025, I have to say it was like a knife through my soul.”
Mr McSweeney gave evidence to the committee as his former boss Sir Keir Starmer faces a critical day for his premiership.
MPs are due to vote on whether there should be a parliamentary sleaze inquiry into the Mandelson vetting row on Tuesday afternoon .
The PM urged Labour backbenchers to “stand together” against a Tory motion to refer him to the Privileges Committee over claims he misled the Commons about the way the former US ambassador’s appointment was handled.
He sought to dismiss the Commons bid to refer him to the committee which ended Boris Johnson’s political career as a “stunt” by political opponents intent on inflicting damage before the May elections.
Sir Keir considered a “wide range of views” when he made the decision to appoint Mandelson as ambassador to the US, Mr McSweeney added, as he hit out at suggestions by some Cabinet ministers they warned against the judgement at the time.
“I have to say, I know that a lot of people now say they told the Prime Minister they were against it at the time,” he said.
“Everything I know about how the Prime Minister works is he will consult widely, he will take a lot of views on, and if everybody else was opposed to this appointment but me, he would not have made an appointment such as that.”
Mr McSweeney also insisted he was not “trying to get a job” for his “hero” Lord Peter Mandelson and that the peer’s appointment as US ambassador was intended to be in the “national interest”.
Asked if he was the first person to put forward Lord Mandelson’s name, Mr McSweeney said: “I think the first person who put Mandelson’s name forward was Mandelson.”
The peer “talked to a lot of people” including members of the press, making it clear he was interested in the job, Mr McSweeney said.
Giving evidence before Mr McSweeney, ex Foreign Office chief Sir Philip told the committee that there were concerns about Mandelson being handed the US ambassador job.
Asked what worries he had before the appointment, he told the Foreign Affairs Committee on Tuesday: “I think it was very much along the lines of what I just said, around the possibility of his known connection to Epstein, causing an issue subsequently.
“Obviously, I didn’t know what was actually going to happen, because Epstein was such a toxic, hot potato subject in US politics itself, including in the election campaign.”
Sir Philip said there was not a dismissive attitude from Downing Street about the security vetting process for Mandelson, but told MPs: “The word I would use is uninterested.
“I think people wanted to know that all the practical steps required for Mandelson to arrive in Washington on or around the inauguration date. It needed to be completed at pace, as it were.”
A large-scale effort to shore up backbench support was under way on Monday night, with former Labour prime minister Gordon Brown also urging the party to back the Prime Minister.
Allies of Sir Keir appeared confident that a mass rebellion was off the cards as he told a packed meeting of the Parliamentary Labour Party (PLP): “When we stick together and fight together we are so much stronger.”
Earlier, Sir Philip told the Foreign Affairs Committee that the previous ambassador to Washington DC, Dame Karen Pierce, had been “blindsided” by Lord Mandelson’s appointment.
He said: “There was at least one tweet from someone close to Trump, reminding people of what Mandelson had said about Trump many years before, when he was president previously.
“I think those around Trump felt blindsided by the announcement at short notice, shall we say, and it was clear also that Karen and her team had done an excellent job in establishing relationships and access to president-elect Trump, and also when he was a candidate.”
It is understood Labour MPs are being whipped to reject the motion after Commons Speaker Sir Lindsay Hoyle allowed a vote on the issue on Tuesday following requests from Tory leader Kemi Badenoch and other senior MPs.
Sir Keir said the Conservatives had put forward “totally baseless” and “absolutely ridiculous” accusations against him and insisted the motion on Tuesday was “pure politics”, adding: “We need to stand together against it.”
The Privileges Committee was responsible for Mr Johnson’s exit from frontline politics after it investigated him for misleading the House over the “partygate” breaches of Covid-19 laws in Downing Street.
He quit as an MP in 2023 before the committee published a report recommending his suspension.
The Prime Minister has been accused of misleading MPs by saying “full due process” was followed in appointing Lord Mandelson, who was given developed vetting status despite failing security checks.
