Updated 8pm

The UK government was thrown into turmoil on Sunday by its own rules on Covid self-isolation just as it controversially prepares to ditch pandemic curbs in England.

Prime Minister Boris Johnson and finance minister Rishi Sunak will work remotely until July 26, officials said, after Health Secretary Sajid Javid on Saturday tested positive for Covid-19.

Javid had a "lengthy" meeting with Johnson on Friday, according to the Sunday Times, and also appeared alongside other ministers in parliament last week. The prime minister nearly died of Covid last year. 

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

But with millions of schoolchildren and workers forced in recent weeks to stay at home under Covid tracing rules, there was an outcry over the announcement, and Downing Street staged a hurried U-turn.

Johnson will remain at the prime minister's country retreat at Chequers northwest of London, where he was staying when contacted by tracing officials from the National Health Service (NHS).

"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 in a video message.

He also defended the plan to reopen England's economy from Monday -- dubbed "freedom day" by some media, although scientists have grave misgivings as daily infection rates top 50,000, behind only Indonesia and Brazil.

"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," the prime minister 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."

Keir Starmer, leader of the main opposition Labour party, said the government was in "chaos" after sending mixed messages about what it expects the public to do from Monday.

"Yet again the Conservatives fixed the rules to benefit themselves and only backtracked when they were found out," he said.

"They robbed the bank, got caught and have now offered to give the money back."

Seated people wait to receive a COVID-19 vaccine at a temporary vaccination site set up at the Tate Modern in London. Photo: AFP

'Pingdemic' 

The government insists that with two-thirds of the adult population now fully vaccinated, the risk can be managed despite rising infections.

Housing Secretary Robert Jenrick told BBC television Monday was still the "logical moment" to replace legal diktats with "personal judgement", thanks to school summer holidays starting this week and the onset of hotter weather. 

But he conceded that the pandemic's current wave may not peak until September.

Jonathan Ashworth, Labour's health spokesman, said the government was being "reckless" with its plans for Monday, echoing many scientists who say the reopening endangers global health.

"We are against opening up without any precautions in place," Ashworth told the BBC, attacking in particular the government's plan to drop a mandate for wearing masks.

Under the plan for England, all restrictions on social mixing and an order to work from home will be lifted. Nightclubs can reopen, and sports stadia, cinemas and theatres can operate at full capacity.

The surge in infections sweeping Britain led to more than 530,000 people being instructed to self-isolate by an NHS app in the week to July 7, according to latest data.

Some companies such as carmaker Nissan have been losing staff en masse after they were pinged by the app -- in a brewing crisis described by UK newspapers as a "pingdemic".

Staff shortages caused by the isolation rules disrupted the London Underground network on Saturday, with one line suspended entirely.

Sunday newspapers carried industry warnings of food shortages if many more staff are forced home.

From August 16, people who are fully vaccinated will no longer need to self-isolate after a close contact, but the government is under pressure to bring that change forward.

Jenrick told Sky News that for now, the fact that Johnson and Sunak were contacted "shows that the system is working".