Johnson has asked British Foreign Secretary Dominic Raab to deputise for him, Xinhua news agency reported a Downing Street spokesman as saying.
The prime minister, 55, was admitted to St Thomas' Hospital in London with "persistent symptoms" on Sunday night, 10 days after testing positive for COVID-19.
The spokesman said the prime minister was moved on the advice of his medical team and is receiving "excellent care."
"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," said a statement.
"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 statement said.
"The PM is receiving excellent care, and thanks all NHS (National Health Service) staff for their hard work and dedication," it added.
Two uniformed police officers were stationed at the entrance to St Thomas' Hospital on Monday evening.
Throughout the day, officers and police vehicles could be seen entering and exiting the central London hospital.
The death toll of those hospitalised in Britain who tested positive for COVID-19 reached 5,373 as of Sunday afternoon, a daily increase of 439, according to the Department of Health and Social Care.
As of Monday morning, the number of confirmed COVID-19 cases in Britain hit 51,608, up 3,802 in the past 24 hours, said the department.
-- BERNAMA
