In a letter to Education Secretary Gillian Keegan, a coalition asked for a statement on what steps were being taken to keep pupils and staff safe.

Crumbling buildings could end up "costing lives" if they are not repaired, according to school leaders' union NAHT.

The others that signed the open letter are the National Education Union (NEU), the NASUWT, Unison, Unite, GMB and Community.

They say it is "truly shocking" that a Department for Education (DfE) report conceded that some buildings were at risk of collapse.

Published in December, it warned: "There is a risk of collapse of one or more blocks in some schools which are at or approaching the end of their designed life-expectancy and structural integrity is impaired.

"The risk predominantly exists in those buildings built in the years 1945 to 1970 which used 'system build' light frame techniques."

The DfE admitted the risk was "worsening" - something the unions say shows the situation has "reached absolute rock bottom".

'Disaster waiting to happen'

Kevin Courtney, NEU general secretary, said it was "disgraceful" that school buildings had been allowed to fall into a dangerous state, and worrying that the government "does not even know which buildings fall into this category".

"In one of the most advanced economies in the world it is shocking that many children, young people and school staff work and learn in an environment that is dangerously unsafe," he said.

Paul Whiteman, general secretary of the NAHT, called it a "disaster waiting to happen, which in the worst-case scenario could end up costing lives".

The unions want a full list of the buildings that could collapse and disclosure of when they will be made safe.

A DfE spokesperson said: "We take the safety of pupils and staff extremely seriously. The department works closely with schools and responsible bodies to ensure all schools buildings are well maintained and safe.

"If the department is made aware of a building that poses an imminent risk of collapsing, immediate action is taken to ensure safety and remediate the situation.

"At present, the department is not aware of any school building that remains open in this state and would expect responsible bodies to immediately approach us if this were the case."