British Prime Minister Boris Johnson 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 evenilng.
The Queen has been kept informed about Mr Johnson’s health by No 10, according to Buckingham Paace.
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.”