Prime Minister Boris Johnson has tested positive for coronavirus.
Mr Johnson said he developed mild symptoms over the past 24 hours, including a temperature and cough.
He said he will now self-isolate in No 10 but will “continue to lead the government’s response via video-conference as we fight this virus.”
Mr Johnson was last seen on Thursday night, as he clapped outside No 10 as part of a nationwide gesture to thank NHS staff.
In a video on his Twitter account, Mr Johnson, 55, said: “I’m working from home and self-isolating and that’s entirely the right thing to do.
“But, be in no doubt that I can continue thanks to the wizardry of modern technology to communicate with all my top team to lead the national fightback against coronavirus.
“I want to thank everybody involved and, of course, our amazing NHS staff.”
“So thank you to everybody who’s doing what I’m doing, working from home to stop the spread of the virus from household to household,” he added.
“That’s the way we’re going to win.”
Mr Johnson was tested at No 10 by NHS staff, on the personal advice of England’s chief medical officer, Professor Chris Whitty, Downing Street said.
He will still be in charge of the government’s handling of the crisis, the statement added. Earlier this week, the prime minister’s spokesman said if Mr Johnson was unwell and unable to work, Foreign Secretary Dominic Raab, as the first secretary of state, was the selected minister to stand in.
It is not known whether Mr Johnson will still be living with his fiancee Carrie Symonds, who is several months pregnant.
Pregnant women are advised to be particularly stringent when following social distancing advice and minimise social contact for up to 12 weeks.
In his Twitter message, Mr Johnson also said: “it was very moving last night to join in that national clap for the NHS.”
Mr Johnson has been seen at several of the government’s televised daily briefings in the past week, where he has appeared alongside senior medical officials to update the country on the virus.
Neither the PM’s senior adviser Dominic Cummings nor Chancellor Rishi Sunak – who Mr Johnson has recently appeared alongside while following social distancing advice – have symptoms or have been tested.
Mr Johnson’s spokesman said all No 10 staff “will, of course, remain two metres apart from him at all times if they were to have any contact.”
No 10 has previously confirmed that if the PM couldn’t work, Foreign Secretary Dominic Raab would take over. At the moment, that’s not happening.
The question now is who the PM has had contact with – and who else will need to be tested.
Measures had already been taken to limit contact in Downing Street. Cabinet earlier this week was largely by conference call and the daily briefing has gone digital too.
However, the PM has been in Parliament in recent days and hosted some senior figures in person for cabinet, including the chief medical officer and the health secretary.
A Buckingham Palace spokesman confirmed the Queen saw Mr Johnson more than two weeks ago, on 11 March.
The pair usually meet weekly for the prime minister’s audience with the Queen, but the most recent meetings have been over the phone.
The Prince of Wales also tested positive for the virus earlier this week.
Prince Charles, 71, is said to be displaying mild symptoms “but otherwise remains in good health,” a spokesman said. He is self-isolating in his residence at Birkhall, on the Balmoral Estate, and continues to work.
Also among the public figures to be self-isolating is Labour’s Angela Rayner, who said on Friday she was showing symptoms of the virus.
There are more than 11,600 confirmed cases of coronavirus in the UK, and 578 people have died.