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.