The health secretary, Matt Hancock, who announced last Friday that he also had the illness, is now out of isolation. Johnson said:
"Although I'm feeling better and I've done my seven days of isolation alas I still have one of the minor symptoms, I still have a temperature. So in accordance with government advice I must continue my isolation until that symptom itself goes. But we're working clearly the whole time on our programme to defeat the virus."
Johnson announced last Friday that he had tested positive for coronavirus. He had felt unwell the day before and took a test in the afternoon.
On Thursday 26 March, he had appeared alongside the chancellor, Rishi Sunak, in Downing Street to take part in the first clap for carers at 8pm. He got the test result at midnight and went into isolation, with the doorways between No 10 and No 12 sealed off.
Hancock is assumed to have self-isolated a full 24 hours before he made his positive diagnosis public on the same day as the PM, meaning he got back to work publicly on Thursday - taking the daily press conference.
The prime minister's official spokesman said Johnson had only mild symptoms of the illness and that he chaired the morning coronavirus meeting with ministers and officials.
"He does continue to have mild symptoms, in particular a temperature," he said. "The guidance is clear that if you still have a high temperature you keep self-isolating until it returns to normal."
He did not give details on how high his temperature was or whether he had been checked over by a doctor.
The UK guidance to isolate for seven days is produced by the chief medical officer and Public Health England. The World Health Organization has said someone could be infectious beyond their symptoms so should isolate for a full two weeks.
"Our guidance ... is very clear on what you need to do and the PM is following it," the spokesman said.
On Friday, the government reinforced its warnings that people should stay at home this weekend even if they are tempted to go further afield because of the forecasted warmer weather, particularly on Sunday.
Johnson said in his video on Twitter:
"I reckon people will be starting to think that this is going on for quite a long time and they would rather be getting out there, particularly if they've got kids in the household. Everybody may be getting a bit stir crazy.
"There maybe just a temptation to get out there, hang out and start to break the regulations. I just urge you not to do that. Please, please stick with the guidance now."
That last sentence sounds like a precursor to "Here is The BBC news. Three people were spotted in Hyde Park today not keeping a mandatory two metres apart, therefore you're all going to have to stay under the collective punishment boot for another month".