The departure deepened a political crisis for Starmer after newly published US documents raised questions about Mandelson's past relationship with the late convicted sex offender Jeffrey Epstein.

McSweeney, Starmer's closest adviser, said he took responsibility for recommending Mandelson's appointment.

"The decision to appoint Peter Mandelson was wrong," McSweeney said in a statement.

"He has damaged our party, our country and trust in politics itself," McSweeney said.

"When asked, I advised the Prime Minister to make that appointment and I take full responsibility for that advice."

Mandelson was sacked last September?over his relationship with?Epstein.

However, the controversy escalated after documents released in the United States suggested Mandelson shared market-sensitive information with Epstein. The documents formed part of a larger trove of files made public in the US.

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The material raised questions about Mandelson's conduct at the time, when he was serving as?Britain's business secretary during the 2008 financial crisis.

Starmer faced mounting criticism — and suggestions that he should also step down?— over his judgment in sending Mandelson to Washington in 2024.

The 48-year-old Irish strategist has been keeping a low profile but is being described by some as "the most powerful man in politics" after playing a key role in Starmer's decisive election victory in July 2024.

He was credited with helping steer Labour toward a more centrist agenda after the left-wing tenure of former leader Jeremy Corbyn. McSweeney was said to have been close to Mandelson.

Mandelson was?an influential figure in British politics and the Labour Party for decades.

He was central to Labour's shift toward the political center in the 1990s, in a similar way to McSweeney, helping former Labour Prime Minister Tony Blair modernize the party and win the 1997 landslide election.

The now-72-year-old was long dubbed the "Prince of Darkness," a term?widely used to describe his reputation as a highly skilled, behind-the-scenes political operator. However, he proved to be?a divisive and controversial figure, and was?​forced out of government twice under Blair over?allegations of misconduct.

McSweeney's resignation came as the Foreign Office said it was reviewing an exit payment to Mandelson, who was sacked over renewed scrutiny of the nature of his contacts with Epstein.

Mandelson received an estimated payout of up to £55,000 (€64,000;?$74,000) after only seven months in the job.

He now faces a?police investigation over potential misconduct in public office.?The Foreign Office said it is also reviewing his severance payment "in light of further information that has now been revealed and the ongoing police investigation."

Mandelson is not accused of any sexual offenses.

Edited by: Zac Crellin