Removing face masks from classrooms in England from mid-May would be “unwise” at a time when Covid-19 infections remain in schools and not everyone has received a vaccine, ministers have been told.

Education Secretary Gavin Williamson said the Government is planning to lift the face mask requirement in secondary schools at step three of its road map out of lockdown, which will be no earlier than May 17.

But union leaders and scientists have called for face coverings to remain in secondary schools and colleges beyond May to ensure pupils, staff, parents and the community are not put at risk of infection.

The plea came as a secondary school in Derbyshire was forced to close its doors after more than 100 pupils and staff tested positive for Covid-19.

Jon Richards, head of education at Unison, which represents school support staff, told the PA news agency: “It would be unwise to abandon a sensible and successful approach on masks this soon.

“The Government should hold off until next month.

“New concerns over Covid variants and some increase in school infections show more caution is needed. It’s better to be safe than sorry and put staff, pupils and the community at risk.”

Earlier this week, five unions representing teachers and support staff – as well as scientists, public health experts and parents – wrote to Mr Williamson to urge him to keep masks in place until at least June 21.

But Prime Minister Boris Johnson is expected to announce on Monday that face coverings will no longer be necessary in secondary school classrooms from mid-May, according to The Telegraph.

Mr Williamson told the newspaper that removing face masks will “hugely improve interactions between teachers and students”.

He added: “As infection rates continue to decline and our vaccination programme rolls out successfully, we plan to remove the requirement for face coverings in the classroom at step three of the road map.”

But Professor Martin McKee, of the London School of Hygiene and Tropical Medicine, who is among the signatories to the letter, warned that removing the requirement from May 17 could lead to more young people getting ill during the summer assessment period in secondary schools and colleges.

He told PA: “Not all adults have been vaccinated and many school teachers are relatively young as well. And we haven’t even started vaccinating adolescents yet.”

Prof McKee, who is also a member of Independent Sage, said: “We know that face coverings do reduce transmission. So they are an important part of our armamentarium in response to Covid.

“The second point is that although cases are going down overall in the country, they are not going down everywhere and we’ve just had an outbreak of 100 cases in Derbyshire in a school.

“So we have been here before last summer when cases were very low and then they came back again. So there’s no room for complacency.”

He added: “As we’re coming up to the assessment season, do we really want children being very ill at that time?”

Wilsthorpe School in Long Eaton, Derbyshire, closed following an outbreak of Covid-19, and assessments for GCSE and A-level students due to take place this week have been postponed until the school reopens next week.

Deborah Lawson, assistant general secretary of the Voice section of the Community union, said: “Although cases of Covid-19 are falling generally, there have been outbreaks in a number of schools recently, involving hundreds of staff and pupils, and there is a risk that relaxing rules and restrictions makes people less careful.”

She added: “Recent surveys have highlighted that wearing a face covering makes the majority of our members who responded feel safer in the classroom, so we have advised the Department for Education that we would like face coverings to remain in use until the evidence indicates they are no longer needed, and in line with other indoor activities, such as shopping.”

School leaders unions’ have called on ministers to set out the evidence behind any relaxation of face mask rules to address concerns about infection risk.

Paul Whiteman, general secretary of the school leaders’ union NAHT, said: “Creating the confidence that this next step is the right thing to do is essential to avoid an unnecessary shock to a system in recovery.

“Parents, pupils and staff will want to understand why removing the requirement for face coverings in classrooms is considered appropriate when it is not for other enclosed spaces.”

On face masks, Dr Mary Bousted, joint general secretary of the National Education Union (NEU), said: “We will be very pleased when they are no longer needed – but we advised the Government to assess the impact of further opening up before removing this measure.

“Schools are doing a very good job of keeping pupils and staff safe and they should be permitted to retain the mask-wearing in the classroom if they think it necessary for reasons such as a rise in local infection rates.”

But MPs and parents have raised concerns about face masks in class disrupting pupils’ learning and wellbeing since they were introduced in March.

Christine Brett, co-founder of parent campaign group UsforThem, said: “We know that masks hamper communication and are having a detrimental effect on children learning.

“The physical, psychological and social impact of children wearing masks has not been assessed by PHE (Public Health England) so we hope that the use of masks in other areas of the school will also be stopped without delay as social distancing measures are being relaxed elsewhere.”