“Although I’m feeling better and I’ve done my seven days of isolation, alas I still have one of the symptoms, a minor symptom, I still have a temperature,” Johnson said in a video message posted on Twitter.
“So in accordance with government advice I must continue my self isolation until that symptom itself goes,” Johnson said.
Johnson announced a week ago that he had tested positive and would be isolating in Downing Street.
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In his address, the prime minister told Britons that they needed to stick to the rules on staying at home unless it was essential to go out, ahead of a weekend when good weather is forecast.
“I just urge you not to do that (go out). Please, please stick with the guidance now,” Johnson said. “This country has made a huge effort, a huge sacrifice, done absolutely brilliantly well in delaying the spread of the virus.
“Let’s stick with it now.”
When asked earlier on Friday if Johnson would be out of isolation on Friday, his health minister, Matt Hancock, said: “I’ve absolutely no idea but what I do know is he’s still working.”
Hancock also tested positive for the coronavirus last week at about the same time as Johnson, but he has since left isolation and is working as scheduled.