Boris Johnson has spent the night in an intensive care unit in a London hospital after his condition worsened.
Downing Street said the Prime Minister was still conscious but might need to be put on a ventilator.
The dramatic turn of events followed a day when officials insisted Mr Johnson was in good spirits and still in charge.
He was admitted to St Thomas’s Hospital on the other side of the River Thames from Parliament yesterday evening.
Number Ten said he was moved to the intensive care unit around 7pm on Monday evening.
Foreign Secretary Dominic Raab has taken charge of the government in his role as First Secretary of State.
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. The PM has asked Foreign Secretary Dominic Raab, who is the First Secretary of State, to deputise for him where necessary. The PM is receiving excellent care, and thanks all NHS staff for their hard work and dedication.
London mayor Sadiq Khan added:
Praying for the Prime Minister’s swift recovery. Guy's and St Thomas' Hospital has some of the finest medical staff in the world, and he couldn't be in safer hands.