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”.
It comes as another 181 people died with the virus in a day, latest government figures showed.
It takes the total number of UK deaths to 759, with 14,579 confirmed cases.
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.
Maybe it was inevitable.
One of the first moments that raised eyebrows in the course of the UK outbreak was when health minister Nadine Dorries came down with coronavirus.
Then, last week, we discovered that some key staff in No 10, including the prime minister’s chief Brexit negotiator David Frost, were self-isolating with suspected symptoms.
A fair number of MPs took themselves off into isolation for fear of having contracted the infection.
Their remaining colleagues were continually ordered to sit far apart on the green benches, before finally, this week, Parliament itself closed early, with no certain date for a return of normal business.
Still, the news this morning that the prime minister himself has contracted coronavirus felt like a shock.
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.
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There are more than 11,600 confirmed cases of coronavirus in the UK, and 578 people have died.
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.
Among the other public figures to be self-isolating is Labour’s Angela Rayner, who said on Friday she was showing symptoms of the virus.
Other world leaders including Canada’s Justin Trudeau and Germany’s Angela Merkel have self-isolated after coming into contact with people who have tested positive for the virus, which causes the disease Covid-19.
Concerns have been raised about whether Brazilian President Jair Bolsonaro may possibly already have the virus. He has twice said that his test came back negative, but has refused to provide proof.
‘Nobody is immune’
Politicians including Labour’s shadow home secretary Diane Abbott shared messages to the PM, wishing him a “speedy recovery”.
Mayor of London Sadiq Khan said it was a reminder that anyone can get the virus, while the SNP’s Westminster leader Ian Blackford said Mr Johnson’s diagnosis “sadly shows nobody is immune”.
Former Chancellor Sajid Javid and ex-Health Secretary Jeremy Hunt both said the whole country was thinking of Mr Johnson, and praised him for his “strong leadership”.
Meanwhile, the president of the European Council, Charles Michel, told Mr Johnson: “Europe wishes you a speedy recovery.”
In other developments:
- Police forces in England and Wales have fined people for ignoring guidance issued to prevent the spread of coronavirus
- The UK’s chief scientific adviser Sir Patrick Vallance has said he expects the number of coronavirus cases in the UK to increase over the next two to three weeks, and then start to gradually decrease due to the current lockdown
- The number of people who have volunteered to help the NHS has reached 700,000. The government originally set a target of 250,000 but increased it to 750,000 after a huge response
- More than 7,000 former nurses and midwives have signed up to return to the profession
- Tesco has said that online shoppers will have their deliveries capped at 80 items from now on, while supermarkets will also use a government database of 1.5 million vulnerable shoppers in England to help prioritise delivery slots
- The Queen’s birthday parade, the Trooping the Colour, will not go ahead in its usual form, and other options are being considered.