"Over the last 24 hours I have developed mild symptoms and tested positive for coronavirus," Johnson said. "I am now self-isolating, but I will continue to lead the government’s response via video-conference as we fight this virus."
A Downing Street spokesman said Johnson, 55, experienced mild symptoms on Thursday - a day after he answered at the prime minister's weekly question-and-answer session in the chamber of the House of Commons.
"The prime minister was tested for coronavirus on the personal advice of England's Chief Medical Officer, Professor Chris Whitty," the spokesman said.
"The test was carried out in No 10 by NHS staff and the result of the test was positive," the spokesman said.
(Reporting by Guy Faulconbridge and Kate Holton)