Boris Johnson has tested positive for coronavirus, reports Sky News.
The prime minster tweeted: "Over the last 24 hours I have developed mild symptoms and tested positive for coronavirus.
"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.