World leaders send Boris Johnson messages of support, after his symptoms worsened on Monday afternoon.
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.
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 tests after announcing 11 days ago that he had the coronavirus. His symptoms included a high temperature and a cough.
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".
After very, very little information was shared today, the prime minister was taken into intensive care at around 19:00 BST.
We've been told he is still conscious, but his condition has worsened over the course of the afternoon.
And he has been moved to intensive care as a precaution in case he needs ventilation to get through this illness.
The statement from Downing Street makes clear he is receiving excellent care and he wants to thank all of the NHS staff.
But something important has changed, and he has felt it necessary to ask his foreign secretary to deputise for him where needs be.
That is a completely different message from what we have heard over the past 18 hours or so, where it was continually "the prime minister is in touch" and "he is in charge" - almost like everything is business as usual.
But clearly being in intensive care changes everything.
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.
Intensive care is where doctors look after the sickest patients.
We do not know the full details of the prime minister's condition, but his admission to ICU is the clearest indication of how ill he is.
Around two-thirds of patients admitted to intensive care with Covid-19 will need sedation and ventilation within 24 hours of arriving.
This is a disease that attacks the lungs and can cause pneumonia and difficulty breathing. The body is left struggling to get enough oxygen into the blood and to the body's vital organs.
Boris Johnson has already being given extra oxygen support.
There is no proven drug treatment for Covid-19, although there are many experimental candidates.
But the cornerstone of the prime minister's care will depend on getting enough oxygen into his body and supporting his other organs while his immune system fights the virus.
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".
Mr Johnson's predecessor, Theresa May, and former Labour leader Jeremy Corbyn both said their thoughts were with him.
Mrs May noted that the "horrific virus does not discriminate".
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.