On Thursday, the Prime Minister held a 35-minute meeting with a group of MPs including Lee Anderson who later tested positive for the virus.
Mr Johnson has no symptoms of Covid-19 and will continue to work from Downing Street, his official spokesperson said.
The PM nearly died during his battle with coronavirus earlier this year.
Intensive care doctors at St Thomas’ Hospital in central London plied Mr Johnson with ‘litres and litres’ of oxygen as his chances of survival balanced on a knife-edge.
News of Mr Johnson’s isolation came one day before the Government’s big relaunch on Monday after the last few days brought political chaos to Downing Street.
Events culminated in the PM’s Svengali Dominic Cummings dramatically carrying his belongings out the front door of No10 in a cardboard box following a brutal reckoning that saw his closest ally Lee Cain fall on his sword, having failed to secure the key role of Mr Johnson’s chief of staff.
Prime Minister Boris Johnson must self isolate after being in touch with someone who has tested positive for Covid-19.
Ashfield MP Mr Anderson – part of Mr Johnson’s Red Wall who helped him win the 2019 general election – wrote on Facebook this evening: ‘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.’
Under Government guidelines, those who come in to contact with people who test positive must isolate for 14 days.
The Prime Minister was informed he had come into contact with an infected person via the NHS Test and Trace app.
A spokesperson for the PM 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.’