UK Prime Minister Boris Johnson has been moved to intensive care in hospital after his coronavirus symptoms worsened, Downing Street has said.
A spokesperson said he was moved on the advice of his medical team and is receiving "excellent care".
Mr Johnson has asked Foreign Secretary Dominic Raab to deputise "where necessary", the spokesperson added.
The prime minister, 55, was admitted to St Thomas' Hospital in London with "persistent symptoms" on Sunday.
A No 10 statement read: "Since Sunday evening, 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 this 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."
Chancellor Rishi Sunak said his thoughts were with the prime minister and his pregnant partner, Carrie Symonds, and that Johnson would "come out of this even stronger".
Labour leader Sir Keir Starmer described it as "terribly sad news".
"All the country's thoughts are with the prime minister and his family during this incredibly difficult time," he added.
Johnson was initially taken to hospital for routine tests after testing positive for coronavirus 10 days ago. His symptoms included a high temperature and a cough.
Earlier on Monday, he tweeted that he was in "good spirits".
Last night, on the advice of my doctor, I went into hospital for some routine tests as I’m still experiencing coronavirus symptoms. I’m in good spirits and keeping in touch with my team, as we work together to fight this virus and keep everyone safe.— Boris Johnson #StayHomeSaveLives (@BorisJohnson) April 6, 2020
Last month, the prime minister's spokesperson said if Johnson was unwell and unable to work, Raab, as the first secretary of state, would stand in.
It comes as the number of coronavirus hospital deaths in the UK reached 5,373 - an increase of 439 in a day.
The Department of Health and Social Care said there were now 51,608 confirmed coronavirus cases.
Scotland's First Minister Nicola Sturgeon said she was "sending [Johnson] every good wish".
Mayor of London Sadiq Khan tweeted that St Thomas' Hospital had "some of the finest medical staff in the world" and that the prime minister "couldn't be in safer hands".