British Prime Minister Keir Starmer’s chief of staff, Morgan McSweeney, resigned Sunday, taking responsibility for advising the appointment of Peter Mandelson as UK ambassador to the U.S. despite Mandelson’s links to Jeffrey Epstein.

The decision has triggered Starmer’s biggest crisis since taking office, especially after newly released U.S. files detailed Mandelson’s relationship with Epstein and prompted a police investigation into alleged misconduct.

McSweeney, a key strategist behind Labour’s rise to power, said the appointment was wrong and had damaged public trust in politics.

Starmer, already struggling with low approval ratings, is facing growing criticism within his own party and calls for him to take personal responsibility, CNBC has reported.

The government now plans to release internal communications related to Mandelson’s appointment, a move that could further complicate Starmer’s leadership and relations with the United States.