Coronavirus: Boris Johnson moved to intensive care
Mr Johnson has asked Foreign Secretary Dominic Raab to deputise "where necessary", a spokesman added.
The prime minister, 55, was admitted to St Thomas' Hospital in London with "persistent symptoms" on Sunday.
The spokesman said he was moved on the advice of his medical team and is receiving "excellent care".
A statement read: "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."
It continued: "The PM is receiving excellent care, and thanks all NHS staff for their hard work and dedication."
Mr Johnson tested positive for coronavirus 10 days ago.
Earlier on Monday, he tweeted that he was in "good spirits".