The admission came ten days after he tested positive for coronavirus. It followed the advice of his doctor.
A Downing Street spokesperson said: “This is a precautionary step, the Prime Minister continues to have persistent symptoms of coronavirus ten days after testing positive for the virus.
“The Prime Minister thanks NHS staff for all of their incredible hard work and urges the public to continue to follow the government’s advice to stay at home, protect the NHS and save lives.”
Mr Johnson has been isolating inside No 11 Downing Street since it was first revealed that he had Covid-19 ten days ago.
Should be remain in hospital, it is expected that Foreign Secretary Dominic Raab will temporarily carry out the functions of Prime Minister.