The armed forces will support the mass Covid testing of school students in England next month, as the government pushes forward with its back to school plan for January.
Ministers have been urged by teaching unions to reconsider the staggered reopening of schools, with educators demanding further action on school safety first.
It comes amid rising cases of Covid-19 across the country, as a new, more infectious strain of the virus takes hold.
The government said UK military personnel would be " on standby" to offer support to secondary schools and colleges across England at the start of the spring term.
A total of 1,500 armed forces personnel are being made available to support the Department for Education and Department for Health and Social Care.
Earlier this month, the government said exam-year students would go back to school as normal after the Christmas holidays - but the majority of secondary school pupils would start the term online to allow headteachers to roll out mass testing of children and staff.
Dr Patrick Roach, general secretary of the NASUWT, wrote to the Education Secretary on Monday demanding further action on school safety.
The letter calls for Gavin Williamson to allow schools to move to remote learning for all pupils, except those deemed to be vulnerable or the children of key workers, in the highest tier areas.
"Delaying the return of pupils to schools and colleges at the start of the spring term will also enable all school and college employers to undertake and consult as required on new risk assessments and ensure that they can be compliant with any new measures or requirements contained in any forthcoming national guidance," it says.
The union is also asking the government to publish new safety guidance in light of the new, more infectious Covid-19 variant, introduce mandatory face coverings within schools and give staff priority access to the vaccine.
Military support will come in the form of "phone advice to institutions needing guidance on the testing process" and help setting up testing facilities, the government said.
Ministers added that "in-person support" would be available "to resolve any issues in the situations where testing would otherwise not be able to go ahead."
The tests themselves will be carried out by students "in the vast majority of cases," Number 10 said, adding it would be done so "under the supervision of a school staff member or volunteer who has been trained for the role."
Education Secretary Gavin Williamson said: "I am grateful to the armed forces personnel, and all the school and college staff, leaders and volunteers working to put testing in place.
"This will help break chains of transmission, fight the virus, and help deliver the national priority of keeping education open for all."