The BBC reports that Downing Street has confirmed the news tonight.
He "continues to have persistent symptoms of coronavirus", a spokeswoman said - including a high temperature.
It was described as a "precautionary step" taken on the advice of his doctor.
The report adds that the prime minister remains in charge of the government and urged people to follow its social distancing advice.
The spokeswoman said it was a precautionary step, “as the prime minister continues to have persistent symptoms of coronavirus ten days after testing positive for the virus."
She added: "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."