British Prime Minister Keir Starmer's chief of staff, Morgan McSweeney, quit on Sunday, saying he took responsibility for advising Starmer to name Peter Mandelson as ambassador to the U.S. despite his known links to Jeffrey Epstein, Reuters reported. 

After new files revealed the depth of the Labour veteran's relationship with the late sex offender, Starmer is facing what is widely seen as the gravest crisis of his 18 months in power over his decision to send Mandelson to Washington in 2024.

The loss of McSweeney, 48, a strategist who was instrumental in Starmer's rise to power, is the latest in a series of setbacks, less than two years after the Labour Party won one of the largest parliamentary majorities in modern British history, according to Reuters.