Boris Johnson has insisted “schools are safe” as the Government faces increasing pressure to review its guidance on pupils in England wearing face coverings between lessons.

But the Prime Minister said the guidance, which currently does not recommend masks being worn in schools, could be altered if the medical advice changed.

It comes after Holyrood’s Education Secretary John Swinney confirmed that secondary schools in Scotland will be given “obligatory guidance” that pupils should wear face coverings in corridors and communal areas.

Elsewhere, London Mayor Sadiq Khan is said to be “moving towards” students in the capital wearing face coverings when they cannot socially distance in schools, a source told the PA news agency.

Asked why ministers were ignoring World Health Organisation (WHO) advice that children aged over 12 should wear masks, Mr Johnson said: “The overwhelming priority is to get all pupils into school.

“And I think that the schools, the teachers, they’ve all done a fantastic job of getting ready and the risk to children’s health, the risk to children’s wellbeing from not being in school, is far greater than the risk from Covid.

“If there are things we have to do to vary the advice on medical grounds, we will, of course, do that.

“But as the chief medical officer, all our scientific advisers, have said, schools are safe.”

Mr Johnson told reporters on Tuesday that the Government will “look at the changing medical evidence as we go on”, adding: “If we need to change the advice then of course we will.”

Teaching unions have urged Westminster to keep the guidance under review as evidence continues to emerge.

Kevin Courtney, joint general secretary of the National Education Union (NEU), said: “We have to stay abreast of the science, so when the World Health Organisation says that children over 12 should wear masks in communal areas at school, that ought to be listened to.

“As things currently stand in England, the NEU believes it should be permissible for staff members and students to wear face masks if they wish to do so.”

Following the decision in Scotland, Geoff Barton, general secretary of the Association of School and College Leaders, said parents, pupils and staff needed reassurance from the Government.

“It would be prudent in the light of the decision in Scotland over face coverings in schools, and the recent guidance by the World Health Organisation, for the Government in Westminster to review its existing guidance that face coverings are not required in schools in England, and provide clear direction one way or another,” he said.

“We have two concerns. First: parents, pupils, and staff, will be anxious about the situation and need reassurance from the Government about the public health basis for its policy over face coverings in England, rather than it being left entirely to schools to explain the Government’s rationale.

“And, second: if there is going to be any U-turn by the Government that it does this sooner rather than later, because the start of the new term is imminent.”

Holyrood’s Education Secretary said on Tuesday that, from August 31, pupils in Scotland should be wearing masks in areas where physical distancing is not possible.

Speaking on the BBC’s Good Morning Scotland radio programme, Mr Swinney said the Scottish Government is going further than the WHO guidance by recommending masks on school buses.

Later on Tuesday, Welsh health minister Vaughan Gething said he hoped to make an announcement on whether school children will be made to wear face coverings by the end of Wednesday.

Masks are currently not recommended for students in Wales, but some local authorities are recommending children wear masks while using school buses.

Some schools have already taken the decision to provide their pupils with face coverings, including Oasis Community Learning, which is responsible for 32,000 children at 52 academies across England.

Founder Steve Chalke said giving secondary school pupils visors and masks for moving in between lessons made schools “as Covid-safe as possible”.

He told PA: “This isn’t about our primary schools, and it’s not about our secondary schools in lessons, and it’s not about the bubbles, it is about transition in schools.

“And we all have a responsibility to one another, as the Prime Minister said, ‘hands, face and space’.

“So this isn’t in rebellion against the Prime Minister, this is trying to enact what Boris has asked us to do. We’re trying to work out the details of the big picture.”