Prime Minister Boris Johnson said on Friday that he had the coronavirus and was suffering mild symptoms, becoming the first leader of a major Western country known to have contracted the virus.

The prime minister, 55, said he would isolate himself in his official residence, 10 Downing Street, but would not relinquish his duties.

On Monday, after resisting harsher measures for more than a week, Mr. Johnson imposed a lockdown on Britain to try to curb the spread of the illness. He continued to meet with advisers and appeared most days at a televised briefing, though not on Thursday.