Nearly one in five councils are telling secondary school pupils to wear face masks, a Government minister has revealed.
Nick Gibb, the schools minister, said around 60 local authorities in England had introduced the measures.
Despite it not being official guidance for over a month, Mr Gibb said "discretion" was given to local public health directors to implement the use of face masks.
"The increasing numbers of children who are isolating is driven mainly by infection rates in the community," he told Sky News. "There is a correlation between the number of children who are asked to self-isolate because they’ve come into contact with somebody Covid and the degree of infection in the community.
"We introduced face coverings for pupils in secondary school classrooms as a temporary measure. We reviewed that over Easter, and from May 17 is no longer a recommendation."
Mr Gibb said the choice to use face masks in schools was only introduced in areas of high infection rates, which he said were driving numbers of positive tests in schools.
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"We do give discretion to directors of public health around the country, particularly in areas of high infection rates, to reintroduce off to maintain that recommendation in classrooms," he added. "In about 60 local authorities with high infection rates around the country, there are recommendations to continue face coverings in classrooms.
"It was a temporary measure. And as I said, the issue isn’t about infections within the school – this is about transmission in the community."
It comes after dozens of secondary schools in England brought the measures back after pupils returned from the half-term break.
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Areas of high infection rates in the North West included Thameside, Cheshire and Oldham as cases of the Delta variant rose across the country.
The Telegraph understands that face coverings are being introduced by local authorities amid concerns about high cases or variants in their area.
The Department for Education has been contacted for comment.