Boris Johnson has been admitted to hospital 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”.
It is not known if Mr Johnson will remain in hospital overnight.
He 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.”
Labour’s shadow health secretary, Jon Asthworth, tweeted: “I send my very best wishes to Boris Johnson and his family tonight for a swift recovery. I know the NHS will deliver brilliant care with exceptional professional. We all look forward to seeing the prime minister healthy and well again.”
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.
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.”
Dr Clarke added: “You would normally expect him to get over it quicker but there are instances where people who are fit and healthy and not over the 70 isolation threshold who do struggle a bit – and we don’t really know why.
“It’s a bit like why we don’t understand why some people don’t get symptoms at all or don’t get infected at all.”
Fears were raised over the spread of COVID-19 at the heart of government when a health minister contracted it after attending a Downing Street reception.
Health Secretary Matt Hancock also last week tested positive for the virus, but is now out of self-isolation having stayed at home for seven days.
He had given an update on the prime minister’s health, telling Sky News’ Sophy Ridge On Sunday: “He’s okay, I’ve been talking to him every day, often several times a day…
“It shows this virus affects different people differently.
“I was lucky, I had two pretty rough days and then I bounced back and some people do get it pretty mildly, and then for others it’s very, very serious and the prime minister is not at that end of the spectrum.”
One of the prime minister’s closest advisers on managing the coronavirus crisis, England’s chief medical officer Professor Chris Whitty, has also been self-isolating with symptoms.
On Sunday it was confirmed another 621 hospital patients had died after contracting coronavirus – taking the UK’s total to 4,934.