LONDON—Boris Johnson said Thursday that he would step down as British prime ministerfollowing a wide-scale rebellion in his party, capping an astonishing fall from grace for a politician who once looked poised to dominate U.K. politics for years.
The step bookends an extraordinary 36 hours in British politics in which more than 50 ministers and senior government aides resigned, leaving the British government in a state of paralysis. Mr. Johnson said he would appoint a new cabinet as he stays in office until a successor has been found, a process that might take some months.