The Department for Education said the portable monitors could be used to identify areas where more air-flow is needed.

Teaching unions have been calling for urgent extra ventilation measures.

They welcomed the pledge, but added any ventilation needs picked up by the monitors must be acted upon.

Most Covid safety restrictions have been relaxed in England's schools. Neither masks, bubble groups nor socially distancing rules are required this coming term.

Many schools have been opening windows and doors to keep air moving around classrooms to lessen the likelihood of Covid outbreaks, but this is easier in warmer weather.

Pupils will be offered twice weekly Covid testing and two tests at school when the autumn term starts to lessen the amount of infection coming into school.

However, those who have come into contact with a confirmed case will no longer have to isolate.

The removal of restrictions has left teachers and some parents concerned that the mass-mixing of pupils may lead to a spike in Covid cases.

Education Secretary Gavin Williamson said: "Providing all schools with CO2 monitors will help them make sure they have the right balance of measures in place, minimising any potential disruption to education and allowing them to focus on world class lessons and catch up for the children who need it.

"By keeping up simple measures such as ventilation and testing, young people can now enjoy more freedom at school and college," he added.

Last minute

The DfE says it is prioritising special schools and alternative provision for the delivery of CO2 monitors, as they are likely to have higher rates of vulnerable pupils.

But a statement on Friday made clear that this £25m batch of CO2 monitors has yet to be fully procured, less than two weeks before many schools return.

However, it said: "All schools and colleges are expected to receive at least partial allocations during the autumn term, enabling all settings to monitor areas where they believe airflow may be weakest."

Geoff Barton, general secretary of the Association of School and College Leaders, said: "In truth, this equipment should have been in place ready for the start of the autumn term, and arguably a lot earlier in the crisis, but it is a case of better late than never.

"Government guidance to schools and colleges on reducing the risk of coronavirus transmission highlights the importance of keeping spaces well ventilated, but doesn't go much further than recommending that windows should be opened to improve natural ventilation.

"This is challenging in the depths of a British winter and does not make for an environment which is conducive to learning. Our understanding is that carbon dioxide monitors will indicate when spaces need ventilating thereby reducing the need to keep windows open all the time."

Kevin Courtney, joint general secretary of the National Education Union, said: "This is a really welcome first step in accepting our argument that funding is needed for good ventilation.

"It follows examples set by administrations in New York City, Scotland and Ireland.

"Sadly, Gavin Williamson failed to get on the front-foot over the summer but the initial investment of £25m in CO2 monitors is welcome now and will start to make a difference. It is vital, though, that government must also commit to supporting schools to address any ventilation problems identified by these monitors."