Prime Minister Boris Johnson has been moved to intensive care after his coronavirus symptoms worsened.
Mr Johnson, 55, was admitted to St Thomas' hospital in central London on Sunday after his coronavirus symptoms persisted for 10 days.
Having been in hospital for tests and observation, his doctors advised that he be admitted to intensive care on Monday evening.
Boris Johnson during the coronavirus outbreak
Boris Johnson during the coronavirus outbreak
In this handout photo provided by Number 10 Downing Street, Britain's Prime Minister Boris Johnson chairs the morning Covid-19 Meeting remotely after self isolating after testing positive for the coronavirus, at 10 Downing Street, London, Saturday, March 28, 2020. The new coronavirus causes mild or moderate symptoms for most people, but for some, especially older adults and people with existing health problems, it can cause more severe illness or death. (Andrew Parsons/10 Downing Street via AP)
ARCHIVO - En esta fotografía del jueves 2 de abril de 2020, el primer ministro británico Boris Johnson aplaude afuera de su casa en el número 11 de Downing Street para elogiar a los héroes locales que combaten el coronavirus, en Londres. (Pippa Fowles/10 Downing Street vía AP, archivo)
In this image taken from video of the TWITTER/@BorisJohnson, Britain's Prime Minister Boris Johnson speaks from self isolation which he has been in since contracting coronavirus, Friday April 3, 2020. British Prime Minister Boris Johnson has been admitted to a hospital with the coronavirus. Johnson’s office says he is being admitted for tests because he still has symptoms 10 days after testing positive for the virus. (TWITTER/@BorisJohnson via AP)
In this image taken from video of the TWITTER/@BorisJohnson, Britain's Prime Minister Boris Johnson announces he has tested positive for the new coronavirus, Friday March 27, 2020. Johnson's office said he was tested after showing mild symptoms and was now self-isolating, but would continue to lead the country's response to COVID-19. (TWITTER/@BorisJohnson via AP)
File photo dated 25/03/2020 of Health Secretary Matt Hancock watching Prime Minister Boris Johnson speak during Prime Minister's Questions in the House of Commons, London, both of them have tested positive for coronavirus.
File photo dated 23/03/2020 of a screen grab of Prime Minister Boris Johnson addressing the nation from 10 Downing Street, London. The Prime Minister has said he has tested positive for coronavirus.
File photo dated 20/03/2020 of Prime Minister Boris Johnson speaking at a media briefing in Downing Street, London, on coronavirus (COVID-19), he says he has tested positive for coronavirus.
File photo dated 20/03/2020 of Prime Minister Boris Johnson (centre), Chancellor Rishi Sunak (left) and Dr Jenny Harries (right) speaking at a media briefing in Downing Street, London, on coronavirus (COVID-19). The Prime Minister has said he has tested positive for coronavirus.
File photo dated 06/03/20 of Prime Minister Boris Johnson during a visit to the Mologic Laboratory in the Bedford technology Park in Bedfordshire, he says he has tested positive for coronavirus.
Prime Minister Boris Johnson outside 10 Downing Street, London, joining in with a national applause for the NHS to show appreciation for all NHS workers who are helping to fight the Coronavirus.
(EDITOR�S NOTE: Image Archived 25/03/2020)
Prime Minister Boris Johnson at one of his daily Downing Street Coronavirus Press Briefings.
Prime Minister Boris Johnson MP has tested positive for coronavirus, Downing Street has announced that Mr Johnson has mild symptoms and will self-isolate in Downing Street. He will still be in charge of the government's handling of the crisis, the statement added. (Photo by Keith Mayhew / SOPA Images/Sipa USA)
Prime Minister Boris Johnson leaves 10 Downing Street, London, for the House of Commons for Prime Minister's Questions.
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Mr Johnson is understood to be conscious and was moved to intensive care at about 7pm as a precaution should he require ventilation to aid his recovery.
Foreign Secretary Dominic Raab will "deputise where necessary", a Number 10 spokesman said.
A Number 10 spokesman said: "Over the course of this afternoon, the condition of the Prime Minister has worsened and, on the advice of his medical team, he has been moved to the intensive Care Unit at the hospital.
"The PM has asked Foreign Secretary Dominic Raab, who is the First Secretary of State, to deputise for him where necessary.
"The PM is receiving excellent care, and thanks all NHS staff for their hard work and dedication."
The Prime Minister's official spokesman said on Monday morning that Mr Johnson had been taken to hospital on the advice of his doctor and as a "precaution", and that it was not an emergency admission.
It came just a few hours after Mr Raab told a press briefing that the PM remained in charge of the Government despite remaining under observation.
At a press conference less than two hours before the decision was made, Mr Raab told a press briefing that the PM remained in charge of the Government despite remaining under observation.
Earlier in the afternoon, Mr Johnson tweeted: "Last night, on the advice of my doctor, I went into hospital for some routine tests as I'm still experiencing coronavirus symptoms. 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."
At the daily Number 10 news conference on Monday Mr Raab faced repeated questions as to how Mr Johnson could cope with demands of the premiership if he was sick enough to require hospital treatment.
Downing Street, which had previously described his symptoms as "mild", switched to describing them as "persistent".
They included a cough as well as a continuing high temperature, 10 days after he first tested positive for the coronavirus.
Mr Raab said he had last spoken to the PM on Saturday – almost 48 hours before being sent to brief the nation his well-being and efforts to tackle Covid-19.
Meanwhile, the latest official figures showed 5,373 patients have died in hospital after testing positive for coronavirus in the UK as of 5pm on Sunday – an increase of 439 on the previous day.
Responding to the news, Labour leader Sir Keir Starmer tweeted: "Terribly sad news. All the country's thoughts are with the Prime Minister and his family during this incredibly difficult time."
And Chancellor Rishi Sunak tweeted: "My thoughts tonight are with @BorisJohnson and @carriesymonds. I know he'll be getting the best care possible and will come out of this even stronger."