Mr Johnson said he developed mild Coronavirus – covid19 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 for the amazing work they do and sacrifice during this coronavirus pandemic.
In a video on his Twitter account, Mr Johnson 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.”
He 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.
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