More teachers should be employed by the government to allow schools to stay open safely if Covid-19 infections rise, the largest teaching union has said.

The National Education Union (NEU) is suggesting using student teachers who have yet to find posts, as well as a “mobilisation of supply staff” - and also says schools need extra space.

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The union’s call came as prime minister Boris Johnson issued a plea to parents to send their children back to the classroom when schools reopen next month.

He said the risk of contracting the coronavirus in schools was “very small”, and pupils faced greater harm by continuing to stay at home.

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But Kevin Courtney, NEU joint general secretary, accused the government of being “negligent in the extreme”. “School staff, parents and pupils are being sorely let down by government because of a lack of a Plan B and of ensuring robust track, trace and test is in place throughout the country,” he said.

Mr Courtney added: “Government should be employing more teachers and seeking extra teaching spaces to allow education to continue in a Covid-secure manner if infections rise.

“This should include employment of student teachers who have finished their courses and not yet found jobs, as well as mobilisation of supply staff.”

He said the NEU agreed with “the Chief Medical officer about the benefits a return to full-time education will have for children and young people’s education and wellbeing”.

But Mr Courtney said: “We believe that it is vital that the government must take every step it can both to allow this wider reopening and to keep the R rate [reproduction rate of the coronavirus] below 1.”

The largest heads’ union is also calling on the government to come up with a plan B for school openings.

Paul Whiteman, NAHT general secretary, said: “We want to engage with government, we want some more advice from government about what to do if the pressure on R comes and what to do if we do need a plan B.

“It seems to be an act of heresy at the moment if you talk about wanting a plan B. It’s not defeatist to prepare for the worst whilst hoping for the best.

“If we do have to experience some form of shutdown going forward, we want to learn from what happened before when we had no time to prepare, and be prepared if it comes again.”

Mr Whiteman said that because there was still anxiety about next week, the government’s back-to-school campaign had “really got to engage with parents” and “make sure all of the measures being taken in school are as secure as they can be”.

Many pupils in England have not been to class since March, when schools were closed except to look after vulnerable children and those of key workers.

Schools in Scotland reopened earlier this month, while those in Northern Ireland will welcome pupils again on Monday. English and Welsh schools will follow suit in September.

Mr Johnson said: “I have previously spoken about the moral duty to reopen schools to all pupils safely, and I would like to thank the school staff who have spent the summer months making classrooms Covid-secure in preparation for a full return in September.

“We have always been guided by our scientific and medical experts, and we now know far more about coronavirus than we did earlier this year.

“As the chief medical officer has said, the risk of contracting Covid-19 in school is very small and it is far more damaging for a child’s development and their health and wellbeing to be away from school any longer.

“This is why it’s vitally important that we get our children back into the classroom to learn and to be with their friends. Nothing will have a greater effect on the life chances of our children than returning to school.”

Former Conservative leader Sir Iain Duncan Smith wrote in The Daily Telegraph that the government “needs to show it governs with a steady hand, based on a clear strategy and message”.

He added: “This battle over schools’ reopening must see the prime minister in the lead, galvanising his inner Churchill.

“It is a fight that, if the government wins, will see the start of an uplift in its fortunes, and win it must.”