United Kingdom Prime Minister Keir Starmer on Tuesday endured one of his toughest days since he was elected in 2024, as lawmakers tried to force an inquiry into his decision to appoint Peter Mandelson as the UK's ambassador to the United States.

Mandelson, who had links to the billionaire financier and convicted sex of fender Jeffrey Epstein, was fired less than a year after his December 2024 appointment, when the extent of his relationship with Epstein became clear. But Starmer has faced criticism over why he picked Mandelson, why he was appointed despite failing his vetting, and whether Parliament was misled over the issue.

On the same day that lawmakers debated whether Parliament's Privileges Committee should investigate Starmer's conduct, his former chief of staff, Morgan McSweeney, and his former foreign office chief, Philip Barton, testified before Parliament's Foreign Affairs Committee about the Mandelson appointment.

McSweeney's testimony was largely helpful to Starmer and included him admitting he had made a "serious mistake" in advising the prime minister to appoint Mandelson, and that he took "full responsibility" for the decision.

He added that Mandelson was chosen because it looked like he would work well with US President Donald Trump.

"I don't think the prime minister would have chosen Mandelson if Kamala Harris had been elected president," he added.

Barton told the committee he had been "worried" about the appointment of Mandelson and had thought his links to Epstein "could be a problem". But he said he did not have an opportunity to tell the prime minister about his concerns.

"There was no space or avenue or mechanism for me to put (my concerns) on the table," he said, potentially highlighting a problem with the process, rather than with the prime minister's decision-making.

He also said he thought normal vetting procedures had been followed.

Ahead of the subsequent debate and vote in Parliament, which was instigated by the Conservative Party and supported by other opposition parties, Starmer said the move amounted to little more than a "stunt" by Conservative Party leader, Kemi Badenoch.

But Badenoch said the prime minister had misled members of Parliament "multiple times" and needed to be held to account.

As the debate began, she called on lawmakers from Starmer's ruling Labour Party to vote against their leader.

"What kind of people are they? Are they people who'll live up to the promises they made about standards, and the rules mattering, or are they people who abandon their promises to be complicit in a cover-up?" she asked.

But with 403 lawmakers in the 650-seat Parliament and a working majority of around 165, Labour members were always going to rally behind their leader, especially with local government elections planned for May 7 and the party needing to present a unified front.