According to the Daily Mail:
The Prime Minister, who was admitted to St Thomas’ Hospital in London last night, was taken to intensive care at 7pm this evening.
He is conscious and has asked Foreign Secretary Dominic Raab to deputise for him.
Mr Johnson has been transferred to intensive care as a precaution should he require a ventilator.
A Number 10 spokesperson said: ‘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.