Prime Minister Boris Johnson has spent the night in intensive care at a central London hospital after his coronavirus symptoms worsened.
Mr Johnson, 55, has been given oxygen but has not been put on a ventilator, cabinet minister Michael Gove said.
Foreign Secretary Dominic Raab has been asked to deputise for the PM. Arriving at No 10 on Tuesday, he said the prime minister was “in very good hands”.
World leaders have sent messages to Mr Johnson wishing him well.
The prime minister, 55, was admitted to St Thomas’ Hospital with “persistent symptoms” on Sunday evening and moved to the intensive care ward on Monday at 19:00 BST.
He was moved as a precaution so he could be close to a ventilator – which takes over the body’s breathing process – BBC political correspondent Chris Mason said.
Mr Gove told BBC Radio 4’s Today programme on Tuesday: “The prime minister’s not on a ventilator. He has received oxygen support.”
If there is any change in his condition “No 10 will ensure the country is updated”, Mr Gove added.
The Queen has been kept informed about Mr Johnson’s health, Buckingham Palace said.
As the first secretary of state, Mr Raab is the minister designated to stand in for Mr Johnson if he is unwell and unable to work.
Mr Raab arrived at No 10 on Tuesday morning and will later chair the government’s daily Covid-19 meeting.
He said earlier 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”.
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”.
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.”
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.
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.
Among those who have sent messages to Mr Johnson was Labour leader Sir Keir Starmer, who 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, US President Donald 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”.
French President Emmanuel Macron said he sent “all my support to Boris Johnson, to his family and to the British people at this difficult moment”.
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.
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