Officials said it was a "precautionary step" but it has raised questions about whether the Conservative leader can still run the country
British Prime Minister Boris Johnson was in "good spirits" on Monday and remained in charge of the government despite his admission to hospital for tests after suffering "persistent symptoms" of coronavirus 10 days after being diagnosed, officials said.
Johnson announced on March 27 that he had tested positive for COVID-19, becoming the most high-profile world leader infected with the disease that has spread rapidly across the globe.
The 55-year-old had been self-isolating in his Downing Street flat but on Sunday evening, still suffering a high temperature and a cough, he was driven to a nearby state-run hospital on the advice of his doctor.
Officials said it was a "precautionary step" but it has raised questions about whether the Conservative leader can still run the country.
"The prime minister had a comfortable night in St Thomas' Hospital in London and is in good spirits. He remains in hospital under observation," Johnson's spokesman said.
The spokesman added when Johnson might be discharged would be "entirely a matter for doctors rather than anyone else."
Foreign Secretary Dominic Raab, who chaired Monday's coronavirus meeting in his absence, told the daily media briefing that Johnson was still "in charge", but that he had not spoken to him for two days.
"He's in charge but he'll continue to take doctors' advice on what to do next and we have a team which... is full-throttle making sure that his directions and his instructions are being implemented," said Raab.
The British government was criticised for initially refusing to follow other European countries in requiring people to stay home as the virus spread.
And Johnson himself said in early March that he was still shaking hands with people.
But two weeks ago he ordered a nationwide lockdown and Britain is now in the grip of a serious outbreak.
Over 50,000 cases and more than 5,000 deaths have been recorded so far, with a latest daily toll of 439.
Health Secretary Matt Hancock and the heir to the throne, Prince Charles, have both been infected with coronavirus, although they have since recovered.
In response to the crisis, Queen Elizabeth II made a rare public address on Sunday night, evoking the spirit of World War II and urging Britons to stay united.
"We will be with our friends again, we will be with our families again, we will meet again," she said.
Housing minister Robert Jenrick told BBC television that the prime minister had been working "phenomenally hard" during the crisis, adding that he would be finding it "very frustrating" to be ill.
Johnson is not known to have any underlying health issues, although he has struggled with his weight, but some questioned if he should have taken more time off.
Junior health minister Nadine Dorries, who also had coronavirus but has recovered, added: "Many with #COVID19 are felled by fatigue/temperature and use isolation to sleep and recover.
"Boris has risked his health and worked every day on our behalf to lead the battle against this vile virus."
Sarah Vine, a newspaper columnist and wife of senior cabinet minister Michael Gove, added: "Boris has worked non-stop throughout his illness -- and now we see the result."
US President Donald Trump said he was "hopeful and sure" Johnson would recover, calling the prime minister "a friend of mine" and a "great leader".
Johnson's pregnant partner, Carrie Symonds, moved out of Downing Street after some staff fell ill. But she said on Saturday she had just spent a week in bed with symptoms, although she has not been tested.
Johnson's spokesman would not confirm a report in The Times newspaper reported that the prime minister had been given oxygen treatment.
"Doctors will be monitoring important vital signs such as oxygen saturations," said Rupert Beale, group leader at the cell biology of infection laboratory at the Francis Crick Institute.
He said they would also check Johnson's blood to "see what the immune response to the virus looks like, and to assess liver and kidney function", and may also perform an electrocardiogram to check the heart.