British Prime Minister Boris Johnson said on Friday he had tested positive for coronavirus and was self-isolating but would still lead the government's response to the outbreak.
Chancellor of the Exchequer Rishi Sunak was also confirmed to have contracted the virus.
A Downing Street spokesman said Johnson, 55, experienced mild symptoms on Thursday - a day after he answered at the prime minister's weekly question-and-answer session in parliament's House of Commons chamber.
"Over the last 24 hours I have developed mild symptoms and tested positive for coronavirus," Johnson said. "I am now self-isolating, but I will continue to lead the government's response via video-conference as we fight this virus."
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 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 delagate 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 and the number of confirmed cases has risen to 11,658.
The UK toll is the seventh-worst in the world, after Italy, Spain, China, Iran, France, and the United States, according to a Reuters tally.
Britain's Prince Charles, the 71-year-old heir to the British throne, tested positive for coronavirus earlier this week.
He is in good health and is now self-isolating at his residence in Scotland with mild symptoms along with his wife Camilla, who tested negative, his office said.