Health secretary Sajid Javid has warned that UK daily Covid-19 cases could rocket to a record 100,000 after restrictions are lifted, warning that the country is about to enter “uncharted territory”.

But Javid announced a shake-up of the self-isolation rules from August 16 in a bid to cut the major disruption that would be caused in workplaces and schools by a surge in cases.

Javid said that any adult who had received both jabs would no longer have to self-isolate if they had been in close contact with a person who had tested positive for Covid-19.

Contacts would be advised to take a PCR test and self-isolate if they tested positive. Those receiving a second dose around August 16 would need to wait two weeks for the vaccine to be fully effective.

Meanwhile, Javid said that all under-18s — an age group that has not routinely been vaccinated — would no longer have to isolate from August 16 if they were in contact with a person who had tested positive.

The move will end the significant disruption in schools caused by pupils having to isolate if someone in their “bubble” contracts the disease.

The Department for Education said on Tuesday that on July 1 there were 561,000 students off school in England because of contacts within or outside school, including 471,000 because of close contacts within school. This was an increase of 225,000 in a week.

Javid said on Tuesday that the number of new daily cases could quadruple during the summer from the current level of about 25,000, meaning that ministers are braced for the virus ripping through the country.

Previously Boris Johnson, prime minister, had said that daily cases could hit 50,000 on July 19, the date at which he hopes to lift almost all remaining restrictions, but Javid expects rates to rise sharply after lockdown measures end.

Javid told the BBC that, thanks to the “huge wall of defence” built by the country’s vaccination programme, the number of deaths from the disease was now only one-thirtieth of those the last time daily cases hit 25,000.

Ministers will decide on July 12 whether to press ahead with the final reopening of the economy on July 19, after considering the latest data, but Johnson and Javid expect it to go ahead.

Javid’s admission that cases could surge over the summer to 100,000 a day compares with a previous record seven-day average of 61,240 on January 1 2021. Unvaccinated people, notably the young, will be particularly affected.

Chris Whitty, chief medical officer, on Monday backed the easing of most restrictions, arguing that it was better to do it over the summer when schools were closed, rather than in the autumn ahead of the flu season.

“At a certain point, you move to the situation where instead of actually averting hospitalisations and deaths, you move over to just delaying them,” he said.

Neil Ferguson, a leading epidemiologist at Imperial College London and government modeller on Covid-19, told the BBC’s Today programme that the lifting of most curbs was “a slight gamble” but justified.

“At the peak of the second wave, 50,000 cases would translate into something like 500 deaths, but that’s going to be much, much lower this time, more like 50 or so.”

But he added: “The challenge is, there’s still the potential of getting very large numbers of cases, so if we get very high numbers of cases a day, 150,000 or 200,000, it could still cause some pressure to the health system.”

Ferguson said some “course correction” might be needed later, while Javid said that powers would remain in place for local health officials to reimpose social distancing rules in an emergency.

The government has contingency plans for compulsory Covid certification at mass events in the event of a winter crisis. Javid said he “hoped” the unlocking planned for July 19 would not be reversed.

Last week Javid, soon after replacing Matt Hancock as health secretary, struck a bullish note, saying there was “no going back”. On Tuesday he said: “I hope not, and that’s certainly not in our plan.”

He added: “There may be a variant that comes out in the future that is vaccine resistant. That means the wall of defence that we have built is no longer there.”

The health secretary has been accused by Labour of being “reckless” in planning to remove the legal requirement to wear a mask in certain settings; Javid said he would continue to cover his face in crowded places such as a packed bus or London Underground train.