Hundreds of schools in England are still at risk of collapse from crumbling concrete, according to previously unpublished figures.

Official data, which the Information Commissioner’s Office (ICO) found had been wrongly withheld by the Conservative government, confirmed the presence of reinforced autoclaved aerated concrete (Raac) at 235 Department for Education (DfE) sites.

Overall, the material was found in 457 government buildings, including schools, hospitals and prisons, by August this year. It is the equivalent of one in 500 sites across the public estate.

Experts have warned that schools forced to evacuate children after finding crumbling concrete could face years in portable buildings and temporary classrooms after the previous government dragged its heels on funding.

The number of schools with Raac, which the Health and Safety Executive has said is now “life expired” and could collapse “with little or no warning”, rose steadily after the Tories announced the sudden closure of more than 100 schools at the end of August last year.

Before the election, the DfE was said to have refused to commit to funding or even give any timescales for starting work. In her budget, Rachel Reeves announced billions of pounds for capital projects including schools.

The money included £1.4bn to fund the existing school rebuilding programme in the 2025-26 financial year, up £550m on this year, at 518 schools and sixth-form colleges across England. But it will not fund any new projects. There was a further £2.1bn to “improve the condition of the school estate”, up £300m on this year.

A DfE spokesperson said: “The government is committed to resolving the problem of Raac as quickly as possible, permanently removing Raac either through grant funding or the school rebuilding programme.”

The figures also show there was crumbling concrete found in 48 Department of Health and Social Care buildings, including hospitals, by August. Raac has been removed from a further 12 sites.

There were 77 Ministry of Defence buildings where Raac had been found and 19 Ministry of Justice sites such as prisons and probation centres, while the Department for Transport had 58 affected buildings, most of them vehicle testing sites.

A Labour source said: “It’s no wonder the shame-faced Conservatives didn’t want the public to know the true scale of the problem when on their watch they allowed our schools, hospitals and prisons to crumble. Labour is investing in our country’s future and will rebuild Britain.”

The ICO found last month that the previous government had been wrong to withhold the Raac data, which was requested under the Freedom of Information Act.

Raac has been ignored by successive governments, but Conservative administrations since 2010 have drastically reduced the budget for school renovations and rebuilds, making the problem more acute.

• This article was amended on 12 November 2024 because the headline and text of an earlier version referred to “primary schools” whereas schools generally was meant.