Mr Johnson was admitted to hospital last night after symptoms of the coronavirus had failed to
He previously tested positive for the disease and had been receiving treatment at St Thomas' Hospital in London.
But a Downing Street spokesman tonight confirmed the PM had been moved into the hospital's intensive care unit at around 7pm.
They said: "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."
It is understood the Prime Minister is conscious, and the move was made in case he needs the use of a ventilator.
Earlier Downing Street, which had previously described his symptoms as “mild”, switched to describing them as “persistent”.
They included a cough as well as a continuing high temperature, 10 days after he first tested positive for the coronavirus.
Chancellor Rishi Sunak tweeted: “My thoughts tonight are with @BorisJohnson and @carriesymonds. I know he’ll be getting the best care possible and will come out of this even stronger.”
And members of the newly-formed shadow cabinet offered their support for the PM.
Shadow justice secretary David Lammy tweeted: “Awful news. My very best wishes to the Prime Minister, as well as his partner Carrie, family and friends. Get well soon @BorisJohnson.”
Labour deputy leader Angela Rayner posted: “Sending my best wishes at this worrying time to the PM @BorisJohnson and @carriesymonds and hoping that his condition stabilises in his battle with #Coronavirus.”
And shadow deputy mental health minister Dr Rosena Allin-Khan tweeted: “I wish the Prime Minister a full and speedy recovery and thank the NHS team looking after him and everyone at St Thomas’.
“Coronavirus can affect anyone, the symptoms can be really awful. Please, please, remind your friends and family to stay at home!”