As the prime minister announces he has coronavirus, concern will turn to the number of people he has come into contact with.
Boris Johnson was last seen in public on Thursday evening as he clapped outside Number 10 as part of a nationwide gesture to thank NHS staff.
He was standing alongside Chancellor Rishi Sunak – both were clearly adhering to the two metre rule.
On Wednesday, Mr Johnson chaired a COVID-19 meeting before appearing at Prime Minister’s Question’s where he was sitting close to Health Secretary Matt Hancock and Home Secretary Priti Patel.
Mr Johnson has held regular cabinet meetings and emergency coronavirus sessions – however most ministers are believed to have dialled in.
He has also been seen at several of the government’s televised daily briefings in where he has appeared alongside senior medical officials including England’s chief medical officer, Professor Chris Whitty and the government’s chief scientific adviser Sir Patrick Vallance.
Downing Street has previously confirmed that Foreign Secretary Dominic Raab would stand in if Mr Johnson was too unwell to continue.
If Mr Raab was also ill, the prime minister has the power to delegate responsibility to any of his ministers.
A Downing Street spokeswoman said Mr Johnson was tested on the personal advice of Professor Whitty.
“In keeping with the guidance, the prime minister is self-isolating in Downing Street,” she added.
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It is not known whether he is staying with his fiancee, Carrie Symonds, who is several months pregnant.
Neither the prime minister’s senior adviser Dominic Cummings nor Mr Sunak have symptoms or have been tested.
The announcement comes just days after Prince Charles confirmed he had tested positive for coronavirus.
Meanwhile, it is understood Mr Johnson last saw the Queen on 11 March when the monarch held her weekly audience with the prime minister at Buckingham Palace.
Their following meeting, held on 18 March, was conducted via telephone as have their subsequent conversations.