Boris Johnson has urged parents, pupils and Tory MPs to be “patient” over calls to end isolation for entire school “bubbles” when lockdown restrictions in England are lifted later this month.

Former party leader Sir Iain Duncan Smith is among 48 MPs to have signed a letter to the Prime Minister warning that the current policy is “disproportionate” and “unsustainable”.

The call came after official data showed that 279,000 children in England are isolating because of possible contact with a Covid-19 case.

Education Secretary Gavin Williamson has suggested that school bubbles will end when classes return after the summer holidays in September.

But the letter said it is essential that schools “go back to normal” when lockdown is lifted – set for July 19 – even if it is “just for the last few days of term”.

“This will send an important signal ahead of the autumn that the route to freedom is a ‘one-way road’ and genuinely ‘irreversible’,” the letter said.

It said pupils have suffered “unnecessary and significant disruptions” to their schooling during the pandemic in order to keep the rest of the country safe.

“They have lost physical fitness, suffered mental health damage, and experienced catastrophic learning loss,” it said.

“Children need normality, security and certainty. If we are to have a hope of levelling up and building back better, we must restore children’s school lives to normal so they can recover their health, wellbeing, education and their futures.”

The Prime Minister said he understands the “frustration” over whole bubbles being sent home to isolate but said the Public Health England review into favouring testing over isolation is still under way.

“They haven’t concluded yet so what I want to do is just to be cautious as we go forward to that natural firebreak of the summer holidays when the risk in schools will greatly diminish and just ask people to be a little bit patient,” he said during a visit to the Nissan plant in Sunderland.

Other signatories include former cabinet minister Esther McVey and Commons Education Select Committee chairman Robert Halfon.

Molly Kingsley, co-founder of the UsforThem campaign, which organised the letter, said she is “delighted” that so many MPs are supporting it.

“Children have been at the bottom of the heap in decision-making for the last 15 months,” she said.

“Children have had enough and now need normality, security and certainty. They need to recover their health and wellbeing, education and their futures, and we owe it to them to now put their interests first.”

Elsewhere, an NHS source said the rules around isolation needed reviewing for medics too.

They said the country was in an “intense period of modification”, adding: “The rules around (self-isolation) need a rethink.

“(We’ve) got lots of medics who are saying ‘I’m double jabbed, I’ve done a flow test, I’m negative, can I come back to work?’”

The deputy chief executive of NHS Providers, Saffron Cordery, said: “Keeping patients and staff safe is an absolute priority for trust leaders, and no-one underestimates the crucial importance of rigorous infection control.

“But we know the continuing self-isolation requirements are a source of frustration for some staff, particularly for those who are double-jabbed, tested negative and want to support their colleagues.

“These rules can be disruptive, so it’s important to ensure they are evidence-based and kept under close review.”