Boris Johnson has been admitted to hospital for tests ten days since confirming he has coronavirus.
Sky News understands the prime minister still has persistent symptoms and a high temperature – and was taken to hospital on the advice of his doctor.
The admission is said to be a precautionary rather than emergency measure. It is not known whether he will remain in hospital overnight.
The prime minister is still leading the government response to the COVID-19 outbreak, it is understood.
Sky News’ political correspondent Tom Rayner said Mr Johnson was taken to an NHS hospital in London earlier on Sunday evening and did not require an ambulance.
GP Dr Ellie Cannon said that still being ill at ten days “is probably a red flag if he still has symptoms”, adding: “Until you are fully better you are not out of the woods.”
Mr Johnson has been self-isolating in 11 Downing Street since testing positive; he uses the residence above the chancellor’s official 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.
Fears were raised over the spread of COVID-19 in Westminster 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 gave an update on the prime minister’s health on Sunday, 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 adviser on managing the coronavirus crisis, England’s chief medical officer Professor Chris Whitty, has also been self-isolating with symptoms.