A spokesman said he was moved on the advice of his medical team and is receiving excellent care.
Mr Johnson had asked foreign secretary, Dominic Raab to deputise where necessary, the spokesman added.
The prime minister, 55, was admitted to hospital in London with persistent symptoms on Sunday evening
The prime minister, 55, was admitted to hospital in London with persistent symptoms on Sunday evening.
A No 10 statement read: “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 [Monday] 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.”
Mr Raab said there was an “incredibly strong team spirit” behind the prime minister.
He added he and his colleagues were making sure they implemented plans Mr Johnson had instructed them to deliver “as soon as possible”.
“That’s the way we’ll bring the whole country through the coronavirus challenge,” he said.