Prime Minister Boris Johnson has spent the night in intensive care at a central London hospital after his coronavirus symptoms worsened.
Mr Johnson, 55, is “in very good hands”, said Foreign Secretary Dominic Raab, who is standing in for the PM and chaired the government’s daily meeting.
World leaders have sent messages to Mr Johnson wishing him well.
It comes as Cabinet Office minister Michael Gove became the latest politician to go into quarantine.
Mr Gove said he was self-isolating at home, after a family member showed mild symptoms. He is not suffering any symptoms and will work from home.
Mr Johnson was admitted to St Thomas’ Hospital with “persistent symptoms” on Sunday and moved to intensive care 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.
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 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”.
Some politicians have called for greater clarity on what Mr Raab’s role as deputy entails, including Tory MP Tobias Ellwood who asked for details “as to where responsibility for UK national security decisions now lies”.
Lord Heseltine, who served as deputy prime minister under John Major, said it will be a “very difficult personal position” for Mr Raab, who “will be tested by the loneliness of the job”.
“He will be surrounded by lots of people who know what Boris Johnson said, believe Boris will be quickly back and have their own personal agendas anyway,” he said.