Prime Minister Boris Johnson has spent 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.
He was moved as a precaution so he could be close to a ventilator – which takes over the body’s breathing process, our correspondent said.
On Tuesday morning, cabinet minister Michael Gove said Mr Johnson was not on a ventilator. If there is any change in his condition “No 10 will ensure the country is updated”, Mr Gove added.
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.”
Arriving at No 10 this morning, Mr Raab – who will later chair the government’s daily Covid-19 meeting – said the prime minister was “in very good hands”.
He previously said there was an “incredibly strong team spirit” behind the prime minister and that he and his colleagues were making sure they implemented plans Mr Johnson had instructed them to deliver “as soon as possible”.
Mr Johnson was initially taken to hospital for tests after announcing 11 days ago that he had the coronavirus. His symptoms included a high temperature and a cough.
Earlier on Monday, he tweeted he was in “good spirits”.
Mr Johnson’s friend and former direction of communications Will Walden told BBC Radio 4’s Today programme Mr Johnson is “far fitter than he looks”.
“He will whip anybody’s backside on a tennis court, he runs regularly, he doesn’t smoke, he drinks moderately.
“So I think if anyone is in a good position both physically and mentally to fight off the disease then the prime minister is that person.”
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”, describing Mr Johnson as “a very good friend of mine and a friend to our nation” who is “strong” and “doesn’t give up”.
Mr Macron said he sent “all my support to Boris Johnson, to his family and to the British people at this difficult moment”.
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.
Former Conservative leader Sir Iain Duncan Smith said the government will “continue to work” as decisions are made collectively by the cabinet.
He also sounded a warning to people who have broken social distancing guidelines, saying “if the most powerful man in Britain can come down with this, so can you”.
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 said there were now 51,608 confirmed coronavirus cases.
Also in hospital with coronavirus is veteran Labour MP for Rochdale Tony Lloyd, 70, who is being treated at Manchester Royal Infirmary.
Chancellor Rishi Sunak said his thoughts were with the prime minister and his pregnant partner, Carrie Symonds, and that Mr Johnson would “come out of this even stronger”.
On Saturday, Ms Symonds said she had spent a week in bed with the main symptoms. She said she had not been tested for the virus.
Scotland’s First Minister Nicola Sturgeon said she was “sending [Mr Johnson] every good wish”, while Northern Ireland’s First Minister Arlene Foster added she was “praying for a full and speedy recovery”.
Wales’ First Minister Mark Drakeford called it “concerning news”.
The Taoiseach – Irish Prime Minister – Leo Varadkar wished Mr Johnson “a rapid return to health”, while European Commission President Ursula von der Leyen also wished him a “speedy and full recovery”.
For Archbishop of Canterbury Justin Welby, the news “deepens our compassion for all who are seriously ill” and those looking after them.
And Mayor of London Sadiq Khan tweeted that St Thomas’ Hospital had “some of the finest medical staff in the world” and that the prime minister “couldn’t be in safer hands”.
During the government’s daily coronavirus briefing earlier on Monday, Mr Raab stressed that the prime minister had been continuing to run the government from hospital.
Asked whether that was appropriate, Mr Raab said Mr Johnson would “take the medical advice that he gets from his doctor”.
“We have a team… that is full throttle making sure that his directions and his instructions are being implemented,” he said.
In other developments:
- A volunteer army of 750,000 people who signed up to support the NHS receives its first tasks
- Experts warn against over-interpreting daily figures of people dying with coronavirus amid reporting delays
- A team of scientists question the impact closing schools has on limiting the spread of coronavirus
- Thousands of people are missed off the government’s high risk list for Covid-19 despite meeting the criteria
- France reports 833 new coronavirus deaths in the past day, the highest daily toll since its outbreak began
- New Zealand’s health minister calls himself an “idiot” after breaking the country’s lockdown by driving his family to the beach.