It was not immediately clear how many Downing Street staff and senior ministers would now need to isolate given that many have had contact with Johnson over recent days and weeks.
When Britain clapped health workers on Thursday evening, Johnson and his finance minister Rishi Sunak came out of separate entrances on Downing Street and did not come into close contact, according to a Reuters photographer at the scene.
Previously the government has said that Johnson had the option to delegate to Foreign Secretary Dominic Raab if needed.
“The prime minister was tested for coronavirus on the personal advice of England’s Chief Medical Officer, Professor Chris Whitty,” the spokesman said.
“The test was carried out in No 10 by NHS staff and the result of the test was positive,” the spokesman said.
So far, 578 people in the United Kingdom have died after testing positive for coronavirus…