A spokesman said he was moved on the advice of his medical team and is receiving “excellent care”.
Mr Johnson has asked Foreign Secretary Dominic Raab to deputise “where necessary”, a spokesman added.
The prime minister, 55, was admitted to hospital in London with “persistent symptoms” on Sunday.
A No 10 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.”
Chancellor Rishi Sunak said his thoughts were with the prime minister and his pregnant partner, Carrie Symonds, and that Mr Johnson would “come out of this even stronger”.
Labour leader Sir Keir Starmer described it as “terribly sad news”.
Mr Johnson was initially taken to hospital for routine tests after testing positive for coronavirus 10 days ago.
His symptoms included a high temperature and a cough.
Earlier on Monday, he tweeted that he was in “good spirits”.
Last month, the prime minister’s spokesman said if Mr Johnson was unwell and unable to work, Mr Raab, as the first secretary of state, would stand in. BBC
