Testing for Covid-19 in schools must improve if they are to remain open, the NEU teaching union has said.

The leaders of Britain’s largest education union have written to Boris Johnson urging the government to take emergency measures if schools and colleges are to stay safe.

They warned the prime minister not to take support from schools for granted, adding that the testing regime needs to be increased and improved.

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The letter, signed by joint general secretaries Dr Mary Bousted and Kevin Courtney, says: “It is now clear that your government has not managed to ensure that testing is sufficiently available to meet the predictable need when 12 million children and their staff returned to school in England.”

Coronavirus: Call for more testing for schools

It outlines problems caused by the current testing system, including a lack of information for schools to stop viral spread, and difficulties children and staff have with obtaining a test, meaning they must remain at home.

The letter continues: “The situation needs addressing as a matter of urgency. Children and staff should be a high priority for the testing regime.

“Indeed, you should move towards asymptomatic testing of staff and older pupils as you have done in the NHS and care homes.”

The letter adds: “School leaders, teachers and support staff have supported the wider opening for all pupils and worked hard to make it as safe as possible, but you cannot, and must not, take this support for granted. ”

The NEU also outlines measures the government should take in the event of a second wave, including a reduction of class sizes, delivery of broadband and laptops to children who do not have them and providing clarity on next year’s exams.

Dr Sebastian Funk, a member of a Scientific Advisory Group for Emergencies (Sage) sub-group, told Times Radio: “I’m very worried about this. I think, to be honest with you, we weren’t really in a place to safely reopen schools.

“I think reopening schools should have been an absolute priority and we should have brought infection levels to lower levels before then. Now we’re at a stage where we can’t even test enough people.

“We’re losing track of what really is going on in the country. At the same time, we are seeing hospitalisations increasing.”

He added: ”I would be very worried indeed about the effect of further openings… the return of universities is, in effect, a reopening.”