Video report by ITV News Political Correspondent Shehab Khan
Boris Johnson had a "comfortable night" at St Thomas' Hospital in London and is still under observation, Downing Street has said.
The Prime Minister tested positive for the virus 10 days ago, but his symptoms "continue to persist" and he was taken to hospital "as a precaution" on Sunday night by "private transportation".
His spokesman has said Mr Johnson is in "good spirits" but wouldn't confirm or deny that he has had oxygen treatment.
His symptoms are described as "persistent" and he has a high temperature and a cough.
"The PM had a comfortable night in St Thomas’s hospital and is and is in good spirits," his spokesman said.
"He remains in hospital under observation.”
Downing Street insisted Mr Johnson will follow doctors’ advice when pressed on whether it was safe for the Prime Minister to continue working while in hospital.
“The PM, as I said, will of course be guided by the advice of doctors,” his official spokesman said.
“The position is that the PM remains in charge of the Government.”
Mr Johnson tweeted he went to hospital for "some routine tests" on the advice of his doctor.
He added: "I’m in good spirits and keeping in touch with my team, as we work together to fight this virus and keep everyone safe.
"I’d like to say thank you to all the brilliant NHS staff taking care of me and others in this difficult time. You are the best of Britain."
Mr Johnson will stay for “as long as needed” in the London NHS hospital where he was taken as a “precautionary step” on the advice of his doctor – rather than as an emergency.
He has been in self-isolation and working from his Downing Street flat since he tested positive for Covid-19 on March 27, with "mild symptoms" including a high temperature and persistent cough.
Mr Johnson will continue to lead the government, despite his hospital admission, a spokesperson said.
But his de facto deputy Dominic Raab chaired the daily Covid-19 meeting on Monday morning for ministers and officials.
The news came just an hour after the Queen delivered a message of hope to the nation amid the Covid-19 pandemic, saying “we will overcome it” although we “may have more still to endure”.
Also on Sunday, a further 621 patients died in UK hospitals after contracting coronavirus, bringing the total to 4,934 – including frontline healthcare workers – according to the latest figures issued by the Department of Health.
“On the advice of his doctor, the Prime Minister has tonight been admitted to hospital for tests," a Downing Street spokesperson said.
“This is a precautionary step, as the Prime Minister continues to have persistent symptoms of coronavirus ten days after testing positive for the virus.
“The Prime Minister thanks NHS staff for all of their incredible hard work and urges the public to continue to follow the Government’s advice to stay at home, protect the NHS and save lives.”
ITV News Political Correspondent Paul Brand explains the latest on Boris Johnson's condition
Mr Johnson has shared several video updates from his Number 11 flat since the diagnosis, and stepped outside to join the nationwide clap for NHS staff on Thursday evening.
He has not been seen publicly since, but spoke to new Labour leader Sir Keir Starmer on Saturday afternoon.
The Prime Minister's pregnant fiancee, Carrie Symonds, also started to suffer from coronavirus symptoms, but she said she was "on the mend."
The 32-year-old who usually lives with Mr Johnson in the Number 11 flat was self-isolating elsewhere.
She tweeted: “I’ve spent the past week in bed with the main symptoms of Coronavirus. I haven’t needed to be tested and, after seven days of rest, I feel stronger and I’m on the mend.
“Being pregnant with Covid-19 is obviously worrying. To other pregnant women, please do read and follow the most up to date guidance which I found to be v reassuring.”
Ms Symonds falls into the group of vulnerable people urged to avoid contact with those with symptoms of Covid-19.
Coronavirus: Everything you need to know