Boris Johnson has tested positive for coronavirus, Sky News reported.
“I am now self-isolating, but I will continue to lead the government’s response via video-conference as we fight this virus.
“Together we will beat this. #StayHomeSaveLives.”
Downing Street said Mr Johnson had been tested on the advice of his chief medical officer Professor Chris Whitty, having been experiencing “mild symptoms” on Thursday.
“The test was carried out in No 10 by NHS staff and the result of the test was positive,” the statement added.
“In keeping with the guidance, the Prime Minister is self-isolating in Downing Street.
“He is continuing to lead the government’s response to coronavirus.”
Mr Johnson, whose partner Carrie Symonds is pregnant, was last seen outside No 10 on Thursday night, joining Chancellor Rishi Sunak as part of a nationwide applause for NHS workers as they battle the pandemic.
He also appeared at PMQs as normal on Wednesday.
Mr Johnson’s spokesman confirmed a few days ago that Foreign Secretary Dominic Raab would stand in if the prime minister became unwell.
If Mr Raab is also ill, the PM has the power to delegate responsibility to any of his ministers, the spokesman added.