New Delhi, Feb. 8 -- British Prime Minister Keir Starmer's chief of staff, Morgan McSweeney, has resigned, intensifying pressure on the Labour leader over the controversial appointment of Peter Mandelson as UK ambassador to the United States.

McSweeney announced his decision in a statement on Sunday (February 8), taking responsibility for advising Starmer to appoint Mandelson - a move now widely seen as the most serious crisis of Starmer's 18 months in power.

In a blunt statement, McSweeney said his resignation was the right course of action.

"The decision to appoint Peter Mandelson was wrong. He has damaged our party, our country and trust in politics itself," McSweeney said.

"When asked, I advised the Prime Minister to make that app...