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.”