Boris Johnson has spoken of “a moral duty” to get all children back in class amid indications he would force pubs, restaurants and shops to close ahead of schools in the event of severe coronavirus flare-ups.

The Prime Minister said schools are the “national priority” and it is understood he only wants them closed as the last resort, after scientific advisers warned more restrictions may be needed to reopen classrooms in England next month.

Children’s commissioner for England Anne Longfield welcomed the pledge after previously criticising the Government for treating pupils as “an afterthought” and calling for schools to be the first to open and last to close.

Schools minister Nick Gibb said this week the Government cannot “decree” that classroom education is prioritised, instead saying decisions will be made by local health chiefs.

But a No 10 source said on Saturday that Mr Johnson’s expectation is that schools will be the last sector to close, with firms being shut first in the event of severe local lockdowns.

“The PM has been clear that businesses including shops, pubs and restaurants should be forced to close first, with schools remaining open for as long as possible,” the source said.

The Association of School and College Leaders (ASCL) said children should have been the priority from the beginning.

Mr Johnson, writing in the Mail On Sunday, said it is the “national priority” to get all pupils back into classrooms in September after months without in-person education.

“This pandemic isn’t over, and the last thing any of us can afford to do is become complacent,” he wrote. “But now that we know enough to reopen schools to all pupils safely, we have a moral duty to do so.”

He warned of the “spiralling economic costs” of parents and carers being unable to work, adding: “Keeping our schools closed a moment longer than absolutely necessary is socially intolerable, economically unsustainable and morally indefensible.”

The Prime Minister was also keen to stress the potential damage to children’s health if they do not return and that the virus presents only a very low threat of making them seriously ill, amid concerns parents may not feel comfortable sending them back during the pandemic.

ASCL general-secretary Geoff Barton said: “Boris Johnson’s article smacks of a Government realising far too late that education should have been their priority throughout and now, Corporal Jones-like, recognising that they are a step behind public opinion.”

Professor Sir Jeremy Farrar, the director of the Wellcome Trust who is advising the Government’s coronavirus response, said the “brief window” before schools reopen must be “used wisely” otherwise new restrictions will be needed.

He wrote in the Observer “there is no such thing as zero risk”, and added: “But it is a risk we can take cautiously, if we take the actions needed over the next month to drive infection rates back down and make schools as safe as possible.

“Most urgently, we need to ramp up testing. We are not where we need to be. We must improve contact tracing, so we’re identifying more cases and providing better, faster data locally.

“If we don’t, we may not be able to reopen schools without introducing new restrictions elsewhere. These are the trade-offs we face – if we do not act now.”

This mirrors a previous comment from Professor Chris Whitty, the chief medical officer for England, who warned the nation has “probably reached near the limit or the limits” of what can be done to reopen society safely.

On Sunday, the children’s commissioner told BBC News: “I’m very pleased this has been stated very clearly and now this needs to be followed through to September to make this a reality.”

Liberal Democrat education spokeswoman Layla Moran said “radical steps” will be needed to open up community spaces to provide space for schools, and to boost contact tracing.

“The Government must also use these last vital weeks to ensure school leaders have the practical and financial support they need ahead of opening fully,” she said.