British Prime Minister Boris Johnson and Health Secretary Matt Hancock are self-isolating after they tested positive for coronavirus.
The pair, who are leading the UK's response to the Covid-19 outbreak, both said they have mild symptoms and are continuing to work from home.
It came as the number of people to have died in the UK after testing positive for coronavirus topped 750.
Mr Johnson, who has a high temperature and persistent cough, said he would still lead the "national fightback" against the virus from his flat above Number 11.
The 55-year-old noticed he had mild symptoms on Thursday afternoon and received the test results at midnight, Downing Street said.
His pregnant partner Carrie Symonds is not believed to be with the prime minister, and is reportedly isolating alone in Camberwell, south London, with the couple's dog, Dilyn.
Mr Johnson and Mr Hancock have been working closely with the country's top medics, including chief medical officer Professor Chris Whitty, deputy chief medical officer Dr Jenny Harries, and chief scientific adviser Patrick Vallance. It is not known whether they have also been tested, but Downing Street said the advice on social distancing had been observed.
In a video message posted to Twitter, Mr Johnson said: "I'm working from home and self-isolating and that's entirely the right thing to do.
"But, be in no doubt that I can continue thanks to the wizardry of modern technology to communicate with all my top team to lead the national fightback against coronavirus."
Mr Hancock tweeted: "Fortunately for me the symptoms so far have been very mild so I've been able to carry on with the work driving forward the UK response."
A spokesman for the chancellor said Rishi Sunak has not had any symptoms and therefore has not been tested for coronavirus and is not self- isolating.
Deaths from coronavirus have surged in Spain, while Italy has passed Chinese infection numbers, as Covid-19 continued to grip Europe.
Spain's Health Ministry reported another 7,800 infections overnight for a total of 64,059. Deaths in Spain climbed by 769 to 4,858 - the world's second highest total after Italy's 9,134.
Spain's severely strained health service has 9,444 workers infected with Covid-19, a figure Amnesty International said is the highest among countries affected by the outbreak. The number is nearly 15pc of Spain's total cases.
The day-on-day increase of infections is slightly lower for the first time since a rapid rise in early March. There was a 8,500 increase reported on Thursday. Spain has the second-highest tally in Europe and fourth in the world.
"It's true that we have more deaths than what we saw yesterday, but it's also true that the percentage increase today is similar to that of the past three days and it appears there is a stabilisation," said Fernando Simon, the head of the health emergency co-ordination centre.
The worldwide death toll climbed to more than 24,000, according to Johns Hopkins University, but more than 124,000 people have recovered, about half in China.
Italy has become the second country to overtake China in coronavirus infections after the US, recording 86,498 on the day it recorded its single biggest leap in deaths, with 969 more victims.
Italian epidemiologists warned that the country's numbers are likely to be much higher than reported - perhaps by five times - although two weeks into a nationwide lockdown the daily increase seems to be slowing, at least in northern Italy.
In France, a 16-year-old student became the youngest person in the country to die from the virus. Her sister said she was admitted to hospital on Monday after developing a "slight cough" last week, and she died on Tuesday in hospital in Paris. "We must stop believing that this only affects the elderly," said the sister. "No one is invincible against this mutant virus."
France has reported more than 1,600 deaths and 29,000 infections, and yesterday extended its nationwide confinement measures for another two weeks.
Meanwhile, New York Governor Andrew Cuomo is seeking 4,000 more temporary hospitals beds across New York City and ordered schools closed for two more weeks.
There are already more than 6,000 hospitalised Covid-19 patients in New York, with almost 1,600 in intensive care. The state has logged a nation-high of 519 deaths, and has more than 44,000 confirmed cases.