Johnson was admitted to hospital on Sunday night and had been undergoing tests after suffering persistent coronavirus symptoms for more than 10 days, but the government had earlier said he was in good spirits and still in charge.
Below is the Downing Street statement:
"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 deputize for him where necessary.The PM is receiving excellent care, and thanks all NHS staff for their hard work and dedication."
Johnson, 55, tested positive for the virus on March 26.A source later said Johnson remained conscious.