The news was confirmed by the Prime Minister’s spokesman this evening after the Prime Minister had been contacted by the NHS Test and Trace service.
The Prime Minister, however, does not have any symptoms of the virus.
It is now expected that he will work from home for the immediate future.
A spokesman said: “The Prime Minister has today been notified by NHS Test and Trace that he is required to self-isolate as a contact of someone who has tested positive for COVID-19.
“The Prime Minister will follow the rules and is self-isolating.
“He will carry on working from Downing Street, including on leading the Government’s response to the coronavirus pandemic.
“The PM is well and does not have any symptoms of COVID-19.”
Mr Johnson met with a small group of Tory MPs on Thursday morning.
Within the group, Lee Anderson, MP for Ashfield later tested positive for coronavirus.
After the meeting, Mr Anderson had posted a picture of himself with the Prime Minister before he received his test result.
He said: “Breakfast with the PM. This morning I met with the PM at Number 10.
“My wife and I both tested positive.
“I feel absolutely fine and my biggest concern is my wife who is in the shielded group. But we are both feeling good.”
No10 has not yet confirmed how the Prime Minister will proceed with Cabinet or indeed Prime Minister’s Questions.
Mr Johnson will, however, follow the rules set out for the public over self-isolating.
Under NHS guidelines, you must self-isolate for 14 days if you have been told to isolate by NHS Test and Trace.
More to follow…