Prime Minister Boris Johnson has tested positive for coronavirus.
Mr Johnson said he had developed mild symptoms over the past 24 hours, including a temperature and cough.
He is self-isolating in Downing Street but said he will “continue to lead the government’s response via video-conference as we fight this virus”.
Following Mr Johnson’s announcement, Health Secretary Matt Hancock also said he had tested positive for the virus.
Mr Hancock said his symptoms were also mild and he was working from home and self-isolating.
Mr Johnson was last seen on Thursday night, clapping outside No 10 as part of a nationwide gesture to thank NHS staff and carers.
He is thought to be the first world leader to announce they have the virus.
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 fight-back 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, Prof Chris Whitty, Downing Street said.
He remains in charge of the government’s handling of the crisis and is understood to have chaired a phone call on Friday morning.
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, would stand in.
Mr Johnson is in the flat above 11 Downing Street, which is now sealed off.
He is working from the office and study in No 11, with meals and work being left at the door of the flat, Mr Johnson’s official spokesman said.
The prime minister’s fiancée Carrie Symonds, who is several months pregnant, is also self-isolating, although it is not known if they are still living together.
Pregnant women are advised to be particularly stringent when following social distancing advice, and minimise social contact for up to 12 weeks.
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”.
His self-isolation will need to last for seven days, according to the advice for people with symptoms.
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.
Also on Friday, Mr Hancock said he had been “working from home over the last couple of days” with mild symptoms, and will be self-isolating until Thursday
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.
The prime minister’s Brexit negotiator, David Frost, is already understood to be off work with suspected symptoms of the virus.
Neither the PM’s senior adviser Dominic Cummings nor Chancellor Rishi Sunak – with whom Mr Johnson has recently appeared alongside, while following social distancing advice – has symptoms. They have not been tested.
A Buckingham Palace spokesman confirmed the Queen, 93, saw Mr Johnson more than two weeks ago on 11 March, and she is in good health.
The pair usually meet weekly for the prime minister’s audience with the Queen, but the most recent meetings have been over the phone.