LONDON - British Prime Minister Keir Starmer said Monday that he will resign as the country's leader and head of its ruling Labour Party following a heavy defeat in May's local elections that triggered a rapid loss of support in his party.

Starmer had said he would stay on and fight a leadership challenge, but he ultimately bowed to mounting calls within the Cabinet to step aside after his rival, the former Mayor of Manchester Andy Burnham, achieved an emphatic victory against the far-right opposition party Reform UK in a by-election held Thursday.

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