Prime Minister Boris Johnson is spending the night in intensive care at a central London hospital after his coronavirus symptoms worsened.
Downing Street said he was moved to the unit on the advice of his medical team and was receiving “excellent care”.
Mr Johnson has asked Foreign Secretary Dominic Raab to deputise “where necessary”, a spokesman added.
The prime minister, 55, was admitted to St Thomas’ Hospital with “persistent symptoms” on Sunday evening.
The Queen has been kept informed about Mr Johnson’s health by No 10, according to Buckingham Palace.
World leaders – including US President Donald Trump and French President Emmanuel Macron – have expressed their support for Mr Johnson.
BBC political correspondent Chris Mason said the prime minister was given oxygen late on Monday afternoon, before being taken to intensive care.
The move means the prime minister is closer to a ventilator – which takes over the body’s breathing process – although it is understood he is not on one.
A No 10 statement read: “The prime minister has been under the care of doctors at St Thomas’ Hospital, in London, after being admitted with persistent symptoms of coronavirus.
“Over the course of [Monday] 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.”
It continued: “The PM is receiving excellent care, and thanks all NHS staff for their hard work and dedication.”
Mr Raab – who will later chair the government’s daily Covid-19 meeting – said there was an “incredibly strong team spirit” behind the prime minister.
He added that he and his colleagues were making sure they implemented plans Mr Johnson had instructed them to deliver “as soon as possible”.
“That’s the way we’ll bring the whole country through the coronavirus challenge,” he said.
Mr Johnson was initially taken to hospital for routine tests after testing positive for coronavirus 10 days ago. His symptoms included a high temperature and a cough.
Earlier on Monday, he tweeted that he was in “good spirits”.
Labour leader Sir Keir Starmer described it as “terribly sad news”.
“All the country’s thoughts are with the prime minister and his family during this incredibly difficult time,” he added.
Meanwhile, Mr Trump said Americans “are all praying for his recovery”.
He described Mr Johnson as “a very good friend of mine and a friend to our nation” who is “strong” and “doesn’t give up”.