Masks should remain in secondary school classrooms in England beyond mid-May to prevent negative “consequences” on the health of pupils and parents, a group of unions and scientists have said.

A joint letter to Education Secretary Gavin Williamson claims that face coverings are “an essential part of the wider system of control in schools” and they should continue to be required until at least June 21.

It came after schools minister Nick Gibb said last week that he hoped that pupils would no longer have to wear face masks in class in secondary schools and colleges in England from mid-May.

But he said the decision on whether to lift the precautionary measure when further easing of social contact limits indoors are confirmed, which will be no earlier than May 17, will depend on “the data”.

The letter, from five unions representing teachers and support staff, as well as scientists, public health experts, parents, highlights concerns about the risks of developing long Covid, warning that an estimated 43,000 children and 114,000 school staff are thought to be suffering from the condition.

We do not want a repeat of past mistakes that previously led to new waves, higher deaths, and prolonged lockdowns

The Department for Education (DfE) has said it expects to remove the requirement on secondary school pupils and staff to wear masks in class as part of the next stage of lifting the lockdown.

Any changes to the policy will be confirmed with one week’s notice following a review of the latest data on infection and vaccination rates.

The joint letter to Mr Williamson says: “To strip these necessary protections, when there are already too few mitigation measures in schools, and when rates of Covid-19 are still significant would have consequences for the health of our children and their parents as well as their communities.”

The letter, from the National Education Union (NEU), the NASUWT teaching union, Unite, Unison and GMB, warns that increasing infection among children puts household members, parents and the wider community at risk.

It came as Office for National Statistics (ONS) figures suggest fewer secondary school pupils and staff in England tested positive for Covid-19 shortly after schools fully reopened in March compared to the autumn term.

Around 0.33% of pupils and 0.32% of staff in secondary schools tested positive for Covid-19 from mid to late March, compared with 1.22% and 1.64% in December, according to the small study of schools.

It is expected that face coverings will no longer be required in classrooms at step 3 of the road map, which will be no earlier than May 17

Dr Deepti Gurdasani, an epidemiologist at Queen Mary University of London, who is among the signatories to the letter, said: “Scientists, school staff, parents and students are alarmed.

“We do not want a repeat of past mistakes that previously led to new waves, higher deaths, and prolonged lockdowns.

“That’s why we’ve come together to urge the Government to consider the global and national evidence on current infection rates in schools.

“Face coverings should be continued in schools after May 17, with review prior to the next stage of the road map on June 21, to avoid the risk of new outbreaks.”

A DfE spokeswoman said: “It is expected that face coverings will no longer be required in classrooms at step three of the road map, which will be no earlier than May 17.

“The lifting of further restrictions at step 3 will follow a review of the latest data on infection and vaccination rates, and all other school safety measures, including regular asymptomatic testing, will remain in place.”