Boris Johnson has one final chance to overhaul a dysfunctional Downing Street operation beset by crises, Conservative MPs have warned after bitter infighting triggered the sudden departure of one of his closest aides.

A string of Tories said the resignation of No 10 communications director Lee Cain, after Johnson’s fiancee and inner circle blocked his promotion to chief of staff, epitomised a No 10 riven by factions, cut off from backbench MPs and lurching from crisis to crisis.