The PM's former chief of staff has said he made "a serious mistake" in recommending the appointment of Lord Mandelson as the UK's ambassador to the US.

Morgan McSweeney, who resigned in February over the advice, said he felt the peer's experience as an EU trade envoy would help the UK secure a US trade deal.

However, he told MPs on the Foreign Affairs Committee that after fresh revelations about Lord Mandelson's friendship with the late convicted sex offender Jeffrey Epstein emerged it "dawned" on him that the peer had not given the "full truth" about their friendship.

McSweeney admitted No 10 wanted Lord Mandelson in post "quickly" but insisted officials were never asked to "skip steps".

He also denied telling the Foreign Office "explicitly or implicitly" that security checks "should be cleared at all costs".

Lord Mandelson was given security clearance for the role by the Foreign Office, despite concerns being raised by vetting officials.

However, the prime minister has said no one in Downing Street - including himself and McSweeney - were aware the Foreign Office had gone against the vetting recommendation until recently.

Lord Mandelson was sacked as ambassador in September 2025, after new information came to light about the depth of his relationship with Epstein.

This included photos of the pair together and supportive emails he sent to Epstein as he faced charges for sex offences in 2008.

McSweeney told the committee that when he saw the revelations "it was like a knife through my soul".

"The nature of the relationship that I understood he had with Epstein was not a close friendship," he said.

"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 [of the appointment]."

Before Lord Mandelson was appointed, a due diligence check - a separate process to the security vetting - was carried out by a team at the Cabinet Office and sent to the prime minister.

This flagged Lord Mandelson's continued relationship with Epstein after his conviction as a potential "reputational risk".

McSweeney was subsequently asked by the PM to send three follow-up questions to Lord Mandelson about his association with Epstein.

While at the time McSweeney said he believed the answers were truthful, he later realised he was not given the "full truth" and that revelations in the Epstein files showed Lord Mandelson was "unfit" for the job.

Challenged over whether it was appropriate for him to ask the follow-up questions, given he was a friend of Lord Mandelson, McSweeney acknowledged that in hindsight it would have been "much better" for "public appearances" if the Cabinet Office's propriety and ethics team had done this.

However, McSweeney added that he was not sure the Cabinet Office "would have got better answers".

Earlier, Sir Philip Barton, the top civil servant at the Foreign Office at the time, told the committee Downing Street had been "uninterested" in the vetting process and the focus was on making sure Lord Mandelson was able to start his job by the time of Donald Trump's inauguration.

This backed up the account of his successor, Sir Olly Robbins, that there was "pressure" from Downing Street to complete vetting quickly and that No 10 had been "dismissive" about the process.

However, Sir Philip denied that this affected the substance of the vetting case.

In response to the claims, McSweeney acknowledged that the PM's private office would have chased the Foreign Office for updates on Lord Mandelson's vetting.

He added that Downing Street wanted the process completed "quickly" so Lord Mandelson was in post by the time of Trump's inauguration.

However, he insisted nobody was asked to "skip steps" in any part of the process.

"Yes we wanted it done quickly but at no point did I witness anyone being dismissive about DV [developed vetting] or national security," he added.

McSweeney denied reports he had sworn at the top civil servant at the Foreign Office while asking him to approve the appointment.

Sir Philip also said he had no recollection of being sworn at by McSweeney.

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