“Over the last 24 hours I have developed mild symptoms and tested positive for coronavirus,” the prime minister wrote on Twitter.
“I am now self-isolating, but I will continue to lead the government’s response via video-conference as we fight this virus.
“Together we will beat this. #StayHomeSaveLives”
Johnson, 55, is the first world leader to have confirmed to have contracted the virus.
Prince Charles, the 71-year-old heir apparent to the British throne, also tested positive for the novel coronavirus this week.
“Thank you to everybody who is doing what I’m doing: working from home to stop the spread of the virus from household to household,” Johnson said in a video message.
“After experiencing mild symptoms yesterday, the prime minister was tested for coronavirus on the personal advice of England’s chief medical officer, Prof. Chris Whitty,” a Downing Street spokesperson said.
“The test was carried out in No. 10 by NHS staff and the result of the test was positive,” the spokesperson added.
“He is continuing to lead the government’s response to coronavirus.”
Downing Street refused to comment on Johnson’s pregnant fiancé, Carrie Symonds, but said the prime minister was self-isolating for seven days.
Dominic Raab, the British foreign secretary, will take over Johnson’s duties should the prime minister become too ill to remain in charge.
The U.K. is currently under a lockdown to prevent the spread of COVID-19. More than 9,500 novel coronavirus cases have been reported in Britain, according to the World Health Organization.
Johnson’s positive test was announced as British police began fining people breaching lockdown rules, less than 24 hours after new laws were brought into force.
Also on Friday, British Health Secretary Matt Hancock revealed he had also tested positive for COVID-19 and is self-isolating at home. The announcement was made hours after Johnson provided an update on his situation.
Angela Rayner, the front-runner in the U.K. Labour party’s deputy leadership contest, also stated on Twitter that she is self-isolating after experiencing symptoms. Duncan Selbie, the chief executive of Public Health England, is also self-isolating with his own symptoms.
The latest figures from the British Department of Health, released on Thursday, showed a total of 578 people who tested positive for COVID-19 in the U.K. have died.