Boris Johnson says he is in "good spirits" after spending the night in hospital with coronavirus.
The PM was taken to St Thomas' Hospital in London on Sunday evening with "persistent symptoms" - including a temperature and a cough - for routine tests.
It comes as the number of coronavirus hospital deaths in the UK reached 5,373 - an increase of 439 in a day.
The Department of Health and Social Care reported 51,608 confirmed cases.
The prime minister, 55, tested positive for coronavirus 10 days ago.
He remains in charge of government, although Foreign Secretary Dominic Raab chaired Monday's coronavirus meeting.
In a tweet, Mr Johnson said he was "keeping in touch with my team as we work together to fight this virus and keep everyone safe".
He also thanked the "brilliant NHS staff" taking care of him and other patients, adding: "You are the best of Britain".
The prime minister's official spokesman said he remained in hospital "under observation", and described Russian reports that Mr Johnson had been placed on a ventilator as "disinformation".
He is continuing to receive updates and briefings in hospital, the spokesman added.
Last month, the prime minister's spokesman said if Mr Johnson was unwell and unable to work, Mr Raab, as the first secretary of state, would stand in.
Earlier, Housing Secretary Robert Jenrick said he hoped the prime minister would be back in Downing Street "as soon as possible".
"He's been working extremely hard leading the government and being constantly updated. That's going to continue," he told BBC Breakfast.
"I'm sure this is very frustrating for him, for somebody like Boris who wants to be hands [on] running the government from the front, but nonetheless he's still very much in charge of the government," he added.
US President Donald Trump is among those who has sent his wishes to Mr Johnson.
"All Americans are praying for him. He's a great friend of mine, a great gentleman and a great leader," Mr Trump said, adding that he was sure the prime minister would be fine because he is "a strong person".
And Labour leader Sir Keir Starmer said he hoped the prime minister had a "speedy recovery".
Health minister Nadine Dorries, who herself tested positive for coronavirus last month, said many of those with the virus would be "felled" by fatigue and a high 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," she said in a tweet.
Meanwhile, the former head of the civil service Lord Kerslake said it may be "sensible" for Mr Johnson to "step back" if he is not well enough to carry out his role for now.
"I think in the end if he's not well, he will have to reflect on this because the job's tough at the best of times and it's doubly tough now," he told the BBC Radio 4's Today programme.