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.
London mayor Sadiq Khan added: