LONDON — Dominic Cummings, the influential strategist who masterminded the Brexit campaign in 2016 and helped vault Boris Johnson into Downing Street, abruptly quit as chief adviser to the prime minister on Friday, after days of fierce infighting over the role of hard-line Brexiteers who surround Mr. Johnson.
Mr. Cummings left his job “with immediate effect,” according to the BBC, which had reported earlier that he would stay on until Christmas. The trigger for his departure was the resignation this week of his close ally, Lee Cain, who had been Mr. Johnson’s communication adviser and had left in a squabble over a new press spokeswoman.
A divisive, sometimes mysterious figure, Mr. Cummings set out to transform the government bureaucracy under Mr. Johnson, amassing more power than any unelected official in recent memory. But he clashed with other officials, who said his uncompromising style poisoned the prime minister’s relations with members of his Conservative Party in Parliament and hurt his popularity with the public.