He developed symptoms over the weekend and will be staying in contact with the rest of the Downing Street team during his quarantine period.
On Friday, Boris Johnson announced on Twitter that he had developed ‘mild symptoms’, and tested positive for coronavirus.
A Downing Street spokesman said: “The test was carried out in No 10 by NHS staff and the result of the test was positive.
“In keeping with the guidance, the Prime Minister is self-isolating in Downing Street.”
Health minister Matt Hancock, subsequently tested positive and the government's chief medical adviser, Chris Whitty, is self-isolating.
What are the symptoms?
The symptoms are: a dry cough, high temperature and shortness of breath.
Look out for flu-like symptoms, such as aches and pains, nasal congestion, runny nose and a sore throat.