Prime Minister Boris Johnson on Sunday urged the public to remain prudent against Covid on the eve of lifting pandemic curbs in England, as he confirmed he is self-isolating until July 26.

"Please, please, please be cautious, and go forward tomorrow into the next step with all the right prudence and respect for other people, and the risks that the disease continues to present," he said in a video message.

Johnson and Finance Minister Rishi Sunak were required to isolate after Health Secretary Sajid Javid confirmed on Saturday he had tested positive for Covid-19.

Initially, a Downing Street spokesperson said both Johnson and Sunak were taking part in a government pilot that enables them to continue working from their offices, while self-isolating outside of work.

But in an update after an outcry over the announcement, the spokesperson reversed position and said neither official was participating in the pilot, and would conduct business remotely.

"We did look briefly at the idea of us taking part in the pilot scheme which allows people to test daily. But I think it's far more important that everybody sticks to the same rule," Johnson said.

Accused by opposition politicians of carving out special rules for himself, Johnson urged everyone to respect a national tracing system as infection rates soar.

He also stands accused by the opposition and by scientists of proceeding recklessly with Monday's reopening in England despite the rapid spread of the Delta coronavirus variant.

But the prime minister defended the plan.

"If we don't do it now, then we'll be opening up in the autumn, the winter months, when the virus has the advantage of the cold weather. We lose the precious firebreak that we get with the school holidays," he said.

"If we don't do it now, we've got to ask ourselves, when will we ever do it? So this is the right moment, but we've got to do it cautiously."