Report in June found lightweight reinforced autoclaved aerated concrete could be present in more than 500 schools

The government has told schools to immediately shut buildings constructed with a form of outdated lightweight concrete unless safety measures are in place.

More than 100 schools built with reinforced autoclaved aerated concrete (RAAC) have been contacted so far, the BBC has reported.

The decision, announced just days before the start of the new school year, followed 鈥渘ew evidence about RAAC鈥, education secretary Gillian Keegan told the broadcaster.

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The Department for Education has told schools that buildings which contain RAAC should close for safety reasons

RAAC was used in system build projects across a range of sectors, including hospitals, between the 1950s and the 1990s but has a design life of around 30 years.

Guidance issued by the Department for Education (DfE) today advised schools to vacate and restrict all spaces confirmed to contain the material

The department said that the spaces should 鈥渞emain out of use until appropriate mitigations are in place, even where they would have been deemed 鈥榥on-critical鈥 previously鈥.

It comes more than two months after a National Audit Office report warned that the risk of injury or death from a school building collapse as 鈥渧ery likely and critical鈥.

The report said the DfE had identified 572 schools where RAAC might be present and that it had been confirmed in 65 buildings, 24 of which required immediate action.

Although the NAO said the DfE had made progress in the last year, it warned the department still 鈥渓acks comprehensive information鈥 on the extent and the severity of safety issues which would allow it to develop a longer term mitigation plan.

>> Also read: How do we fix England鈥檚 crumbling school estate?

Keegan said: 鈥淲e must take a cautious approach because that is the right thing to do for both pupils and staff,鈥 she said.

鈥淭he plan we have set out will minimise the impact on pupil learning and provide schools with the right funding and support they need to put mitigations in place to deal with RAAC.鈥

RAAC is causing increasing concern across the public sector. In June, the government ordered all departments to investigate their estates to identify where it might be present.

In May, the Ministry of Defence revealed it was investigating hundreds of buildings feared to be on the verge of collapse because they were built with the concrete.

Health minister Maria Caulfield also admitted last year that 34 hospital buildings in England were at risk, a situation which was described by the boss of one affected hospital boss as a 鈥渢icking time bomb鈥.