Boris Johnson has been admitted to a hospital in central London for tests 10 days after confirming he had contracted coronavirus.
The prime minister, 55, still has persistent symptoms and went on the advice of his doctor, Downing Street said.
A spokesperson confirmed his admission was a precautionary rather than emergency measure and added he "thanks NHS staff for all of their incredible hard work".
They also urged the public to "continue to follow the government's advice to stay at home, protect the NHS and save lives".
Sky News understands that Mr Johnson will remain in hospital overnight.
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The prime minister travelled, without the need for an ambulance, to an NHS hospital in London at 8pm on Sunday and is understood to still be leading the government response to the COVID-19 outbreak.
GP Dr Ellie Cannon told Sky News that still being ill at 10 days "is probably a red flag if he still has symptoms", adding: "Until you are fully better you are not out of the woods."
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Politicians from all sides have offered messages of sympathy to Mr Johnson.
At the start of his regular White House news briefing on COVID-19, US president Donald Trump said America sends its best wishes to Mr Johnson.
"He's a great friend of mine," Mr Trump said. "I'm sure he is going to be fine, he's a strong man, a strong person."
Labour's new leader Sir Keir Starmer tweeted: "Wishing the Prime Minister well and a speedy recovery."
Labour MP David Lammy wrote: "Get well soon @BorisJohnson. The whole country wants you to return to full health as soon as possible."
SNP leader Nicola Sturgeon wished the PM "all the best and a speedy recovery" while Lib Dem MP Layla Moran tweeted "Wishing ?@BorisJohnson? a speedy recovery" and encouraged people to give the Mr Johnson time to recover.
Labour MP Jess Phillips also sent her regards to Mr Johnson and his fiancee Carrie Symonds, who is pregnant with the couple's first child.
Ms Symonds, 32, announced on Saturday she had spent a week in bed with coronavirus symptoms but struck an optimistic tone, saying: "I haven't needed to be tested and, after seven days of rest, I feel stronger and I'm on the mend."
Mr Johnson has been self-isolating in 11 Downing Street since testing positive; he uses the residence above the chancellor's office as several prime ministers have in the past given it has a larger living space.
He posted a video message on Friday urging people to "stick with" the lockdown measures.
Dominic Raab, the foreign secretary and designated minister who will take over if the prime minister is incapacitated, will chair the government's daily emergency coronavirus committee meeting known as C-19 on Monday morning.
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Dr Simon Clarke, a microbiologist from University of Reading, said after the prime minister was taken to hospital that "this happens to some people".
"We have a seven-day quarantine period usually for people who have the virus, but only if you're feeling better and some people may take a bit longer," he explained to Sky News.
"They will look at his general physiology and biochemistry, make sure his organs are functioning properly and that he can breathe okay.
"I imagine these are fairly routine checks just to make sure he's alright."
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