UK Prime Minister Boris Johnson has been moved to intensive care in hospital after his coronavirus symptoms "worsened", Downing Street has said.
The prime minister, 55, was admitted to hospital in London with "persistent symptoms" on Sunday evening.
BBC political correspondent Chris Mason said the prime minister was given oxygen late on Monday afternoon, before being taken to intensive care.
However, he has not been put on a ventilator.
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."
Mr Johnson has asked Foreign Secretary Dominic Raab to deputise "where necessary", the spokesman added.
The Queen has been kept informed about Mr Johnson's health by No 10, according to Buckingham Palace.
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.
Asked whether that was appropriate, Mr Raab said Mr Johnson would "take the medical advice that he gets from his doctor".
"We have a team... that is full throttle making sure that his directions and his instructions are being implemented," he said.
US President Donald Trump, who said he has twice tested negative for COVID-19, has offered words of support for the prime minister during White House press briefings on the virus.
Trump said of Johnson on Monday afternoon that he was “very saddened to hear that he was taken into intensive care a little while ago.”
“He’s been really something very special,” Trump said. “Strong, resolute, he doesn’t quit, doesn’t give up.”
“But when you get brought into intensive care, that gets very very serious,” Trump added.
The virus, though, had already penetrated the British government.
Johnson and his health minister tested positive last month and his chief medical adviser also self-isolated. Johnson's pregnant 32-year-old fiancee, Carrie Symonds, also had symptoms but said on Saturday she was feeling better.