Prime Minister Boris Johnson has tested positive for coronavirus and is self-isolating in Downing St.
Mr Johnson revealed the news in a tweet after developing symptoms in the last 24 hours but insisted he would continue to lead the government through the crisis.
The PM chaired this morning's Covid-19 meeting via videolink from his flat above Number 11.
It raises fears that other key Downing St staff may have been exposed - and prominent members of the Cabinet.
Health Secretary Matt Hancock also revealed this morning that he developed mild symptoms of the virus and has entered isolation.
He said in a video message that he would be self-isolating until next Thursday.
"Fortunately for me the symptoms so far have been very mild so I've been able to carry on with the work driving forward the UK response.
He also said a "massive thank you to everybody in the NHS, working in social care and right across the board on the response".
No 10 staff do not have to self-isolate unless they show symptoms and a Downing St spokesman said he was "not aware" of any further testing of Cabinet ministers.
Anyone showing signs of coronavirus, which include a temperature and a persistent dry cough, have been instructed to self-isolate for seven days, and everyone living in their household should do the same for a two-week period.
Mr Johnson received his test results at midnight last night after being tested late yesterday afternoon.
He will self-isolate for seven days in No 11 Downing St, normally home to the Chancellor Rishi Sunak, where a video conferencing system has been installed.
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A spokesman confirmed that the doors between No 10 and No 11 would be closed and Downing Street would follow the strict guidelines on how they would interact with the PM.
The spokesman said meals and work would be left at the door of the Prime Minister's flat.
He added: "Number 11 in its entirety will serve as the Prime Minister's office and his home - he lives above Number 11 in the flat.
"The doors between Number 10 and Number 11 have been closed off to all other staff who work in the building.
"The PM will work from the office and the study in Number 11, which has been kindly vacated by the Chancellor.
"Full video conferencing facilities have been installed in those downstairs rooms in Number 11.
"All Number 10 staff will observe Public Health England guidelines on contact with the PM and will of course remain two metres apart from him at all times if they were to have any contact."
Last week the Government confirmed that Foreign Secretary Dominic Raab would take over from the PM if at any point he was incapacitated.
In a video message Mr Johnson said: "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."
Mr Johnson, 55, posted a video on Twitter to say that people should "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".
It is not known whether he is staying with his fiancee, Carrie Symonds, who is several months pregnant.
Ms Symonds falls into the group of vulnerable people urged to avoid contact with those with symptoms of Covid-19.
Downing St would not comment on if Ms Symonds was with Mr Johnson in the No 11 flat, but it is understood that she is not.
Ms Symonds, who is expecting the couple's baby in the summer, is also advised to take social distancing seriously.
While pregnant women do not appear more likely to contract coronavirus than the general population, pregnancy itself alters the body's immune system and response to viral infections in general.
Guidance updated on Thursday from the Royal College of Obstetricians and Gynaecologists (RCOG) says viral infections can "occasionally be related to more severe symptoms and this will be the same for Covid-19".
It says that, while the risks are small overall, health professionals should look out for more severe symptoms of Covid-19 in pregnant women who test positive, such as pneumonia and a lack of oxygen.
But the RCOG says the current expert opinion is that unborn babies are unlikely to be exposed to Covid-19 during pregnancy.
It is thought Mr Johnson last saw the Queen on March 11 when the monarch held her weekly audience with the Prime Minister at Buckingham Palace.
Their following meeting, held on March 18, was conducted via telephone as have their subsequent conversations.
A Buckingham Palace spokesman confirmed the Queen last saw Mr Johnson just over two weeks ago.
He said: "Her Majesty the Queen remains in good health. The Queen last saw the Prime Minister on the 11th March and is following all the appropriate advice with regards to her welfare."
Rishi Sunak, is not self-isolating, a Treasury source said.
When Britain clapped health workers on Thursday evening, Mr Johnson and Mr Sunak came out of separate entrances on Downing Street and did not come into close contact, according to a photographer at the scene.
Confirming the news, a Downing Street spokesperson said: “After experiencing mild symptoms yesterday, the Prime Minister was tested for coronavirus on the personal advice of England’s Chief Medical Officer, Professor Chris Whitty.
“The test was carried out in No 10 by NHS staff and the result of the test was positive.
“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 has been leading the government's response - including hosting daily press conference alongside key cabinet ministers.
It is not known if the Government is testing those who have come into contact with Mr Johnson in the last week to text their exposure to the virus.
Labour leader Jeremy Corbyn led messages from across the political divide.
He tweeted: "I wish the Prime Minister a speedy recovery and hope his family are safe and healthy. Coronavirus can and does affect anyone. Everyone be safe. Our own health depends on everybody else."
Shadow home secretary Diane Abbott added: "I don't wish anyone ill. I wish the Prime Minister a speedy recovery. I hope everyone in coming days gets the care and medical attention they need."
Mayor of London Sadiq Khan posted: "Sorry to hear this and hope you feel better soon. Thank you for everything your Government is doing to help us fight this. This is a reminder that anyone can get £COVID19. We must all follow the rules and stay at home to support our amazing NHS staff to save lives."
SNP Westminster leader Ian Blackford tweeted: "I am sorry to read this & I hope @BorisJohnson makes a speedy recovery. This sadly shows nobody is immune. Everybody must follow the advise on social distancing, non essential work must not take place. We need to take all measures to isolate if need be."
Former chancellor Sajid Javid wrote: "Whole Javid family and country thinking of you @BorisJohnson & @carriesymonds. Thank you for your strong leadership at a time of crisis. Get well soon!"
Jeremy Hunt, Matt Hancock's predecessor as health secretary, posted: "Thoughts and prayers of whole nation with @BorisJohnson having tested positive for coronavirus.
"We are so grateful for your strong leadership at this time of crisis and know you will back with your trademark optimism before long! Thinking of you and Carrie."
Earlier on Friday, Labour deputy leadership candidate Angela Rayner confirmed she has quarantined herself after developing Covid-19 symptoms.
Ms Rayner posted on Twitter: "I am now in self isolation as I have the symptoms of coronavirus - they started last night and have gradually got worse.
"I will be a bit quieter than usual but will still be keeping in touch with people as best as I can."
The timing of the Ashton-under-Lyne MP's isolation means she will be permitted to leave her house by the time the leadership result is announced next Saturday.
Mr Johnson was last seen on Thursday night standing outside No 10 as part of a nationwide thank you to NHS staff.
There are more than 11,600 confirmed cases of coronavirus cases in the UK, and 578 people have died.
Prince Charles also tested positive for coronavirus this week, but the 71-year-old, "remains in good health".
BORIS JOHNSON'S CORONAVIRUS ANNOUNCEMENT IN FULL
"Hi Folks
"I want to bring you up to speed on something that's happening today which is that I've developed mild symptoms of the coronavirus. That's to say - a temperature and a persistent cough.
"And, on the advice of the Chief Medical Officer, I've taken a test. That has come out positive.
"So I am working from home. I'm 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.
"And I want to thank everybody who's involved, I want to thank, above all, our amazing NHS staff. It was very moving last night to join in that national clap for the NHS.
"But it's not just the NHS, it's our police, our social care workers, teachers, everybody who works in schools, DWP staff.
"An amazing national effort by the public services. But also by every member of the British public who's volunteering, an incredible response - 600,000 people have volunteered to take part in a great national effort to protect people from the consequences of coronavirus - I want to thank you.
"I want to thank everybody who's working to keep our country going through this epidemic.
"And we will get through it.
"And the way we're going to get through it is, of course, by applying the measures that you'll have heard so much about.
"And the more effectively we all comply with those measures, the faster our country will come through this epidemic and the faster we'll bounce back.
"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.
"That's the way we're going to win, we're going to beat it, and we're going to beat it together.
"Stay at home, protect the NHS, and save lives."
