The Prime Minister has been told to avoid others after Tory MP Lee Anderson tested positive for coronavirus.
Mr Johnson had held a meeting with the Ashfield MP in Downing Street three days ago, where the pair also had breakfast.
The PM’s spokesman said: "The Prime Minister Boris Johnson 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.”
In a statement after his diagnosis was revealed, Mr Anderson explained he felt fine and his concerns were for his wife, Sinead.
He said: “Isolating.
“On Friday I lost my sense of taste, at the same time my wife had a bad headache. I had no cough, no fever and felt well. We both had a test on Saturday and the result came in Sunday morning.
“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.”
Sinead suffers from cystic fibrosis, diabetes and had a double lung transplant so is in the at-risk category.
Mr Anderson has previously praised her bravery in Parliament, and urged the UK Government to make a cystic fibrosis wonder drug on the NHS.
Posting on Facebook at the time of his meeting with the PM, the Tory backbencher had shared a photo of the pair standing apart in Downing Street.
He said: "This morning I met with the PM at Number 10.
”I was there with my wish list for Ashfield and Eastwood.
"Investment is coming, you have my word on that.”
In April earlier this year the PM went into intensive care with the virus, just ten days after testing positive.
In that instance, Foreign Secretary Dominic Raab deputised for the PM while he was recovering.