Prime Minister Boris Johnson has been admitted to intensive care after his coronavirus symptoms worsened.
Mr Johnson is understood to be conscious and was moved to intensive care at about 7pm on Monday as a precaution should he require a ventilator.
The 55-year-old had been admitted to St Thomas' hospital in central London on Sunday evening after testing positive for Covid-19 10 days ago.
Having been in hospital for tests and observation, his doctors advised that he be admitted to intensive care on Monday evening.
Foreign Secretary Dominic Raab, who last spoke to Mr Johnson on Saturday, will deputise for the PM.
Buckingham Palace said the Queen was being kept informed by Downing Street about the condition of Mr Johnson – the 14th prime minister of her reign.
In a statement recorded after Mr Johnson's admission, Mr Raab said: "The Government's business will continue.
"The Prime Minister is in safe hands with that brilliant team at St Thomas' Hspital, and the focus of the Government will continue to be on making sure that the Prime Minister's direction, all the plans for making sure that we can defeat coronavirus and can pull the country through this challenge, will be taken forward."
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 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.
His move to intensive care 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.
Earlier on Monday 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.
Mr Raab said he had last spoken to the PM on Saturday – almost 48 hours before being sent to brief the nation about his well-being and efforts to tackle Covid-19.
Mr Johnson's fiancee Carrie Symonds previously said she had been suffering Covid-19 symptoms, but is "on the mend".
Ms Symonds, 32, who is expecting the couple's first baby in early summer, said she was not tested for the virus.
Health Secretary Matt Hancock is already back at work after having tested positive for Coronavirus.
And Chief Medical Officer Professor Chris Whitty returned to work on Monday after developing symptoms and the Prime Minister's chief adviser, Dominic Cummings, has also self isolated.
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."
Sir Lindsay Hoyle, Speaker of the House of Commons, said: "This is terrible news. I know the thoughts and prayers of everyone across the House are with the Prime Minister and his family right now.
"We all wish him a speedy recovery."
Fellow world leaders also wished the PM well, including French president Emmanuel Macron who tweeted that his thoughts were with Mr Johnson, his family and the British people "at this difficult time".
And Archbishop of Canterbury Justin Welby tweeted: "The news that our Prime Minister has been moved to intensive care deepens our compassion for all who are seriously ill and for those caring for them. I invite all people of faith to join me in praying for @BorisJohnson and his loved ones."
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.