British Prime Minister Keir Starmer, right, with Britain`s former ambassador to the United States, Peter Mandelson, in February 2025 in Washington. Carl Court/Pool Photo via AP

British Prime Minister Keir Starmer`s chief of staff resigned Sunday over the furor surrounding the appointment of Peter Mandelson as the U.K. ambassador to the U.S. despite his ties to Jeffrey Epstein.

Morgan McSweeney said he took responsibility for advising Starmer to appoint Mandelson, 72, to Britain`s most important diplomatic post in 2024.

"The decision to appoint Peter Mandelson was wrong. He has damaged our party, our country and trust in politics itself," McSweeney said in a statement. "When asked, I advised the Prime Minister to make that appointment and I take full responsibility for that advice."

Starmer is facing a political storm and questions about his judgment after newly published documents, part of a huge trove of Epstein files made public in the United States, suggested that Mandelson sent market-sensitive information to the convicted sex offender when he was the U.K. government`s business secretary during the 2008 financial crisis.

The prime minister apologized last week for "having believed Mandelson`s lies." He said "none of us knew the depth of the darkness" of the relationship between Mandelson and Epstein when the former was vetted for the diplomat job.

"Mandelson betrayed our country, our Parliament and my party ... he lied repeatedly to my team when asked about his relationship with Epstein before and during his tenure as ambassador. I regret appointing him," Starmer told lawmakers last week.

Starmer`s government has promised to release its own emails and other documentation related to Mandelson`s appointment, which it says will show that Mandelson misled officials.

A number of lawmakers have called for Starmer to resign.

"Keir Starmer has to take responsibility for his own terrible decisions," said Kemi Badenoch, leader of the opposition Conservative Party.

Mandelson, a former Cabinet minister, ambassador and elder statesman of the governing Labour Party, has not been arrested or charged.

The Metropolitan Police is now investigating and officers searched Mandelson`s London home and another property linked to him on Friday. Police said the investigation is complex and will require "a significant amount of further evidence gathering and analysis."

Starmer had fired Mandelson in September from his ambassadorial job over earlier revelations about his Epstein ties. But critics say the emails recently published by the U.S. Justice Department have brought serious concerns about Starmer`s judgment to the fore. They argue that he should have known better than to appoint Mandelson in the first place.

The new revelations include documents suggesting Mandelson shared sensitive government information with Epstein after the 2008 global financial crisis. They also include records of payments totaling $75,000 in 2003 and 2004 from Epstein to accounts linked to Mandelson or his husband, Reinaldo Avila da Silva.

Aside from his association with Epstein, Mandelson previously had to resign twice from senior government posts because of scandals over money or ethics.