Scores of schools in England must shut buildings made with an aerated concrete that is prone to collapse, the UK government announced Thursday, just days before the new term starts.

From the 1950s to the mid-1990s, Reinforced Autoclaved Aerated Concrete (RAAC) was widely used for school and college constructions, according to the education ministry.

After officials determined the potential collapse risk of the material over time, the ministry began warning schools in 2018 that "mitigations " were required to reduce the danger.

However, as many as 104 schools and colleges with buildings containing RAAC but "without mitigations in place" have been contacted about closing, the education ministry said in new guidance.

It requested they "vacate spaces or buildings that are known to contain RAAC" because officials had lost confidence that they could remain open without the measures to reduce the collapse risk.

Most will still operate face-to-face learning on their existing sites where RAAC is not present, but "a minority" will need to either fully or partially relocate to alternative accommodation, the ministry said.

It comes as millions of pupils in England are returning to school after the lengthy summer holidays.

"Nothing is more important than making sure children and staff are safe in schools and colleges, which is why we are acting on new evidence about RAAC now, ahead of the start of term," Education Secretary Gillian Keegan said.

"The 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," she said.

But education officials, public-sector unions and opposition parties hit out at the government's handling of the issue.

"This situation is nothing short of a scandal," the Unison union's head of education Mike Short said.

The government had "squandered valuable months hiding this crisis when they should have been fixing dangerous school buildings", he said, warning that the decision would create "turmoil for thousands of families".

The Association of School and College Leaders noted that the dangers of RAAC-related structural failures in schools with it had been known since at least 2018.

"It has taken the government far too long to act on a risk of this seriousness," the ASCL's policy director Julie McCulloch said.

Munira Wilson, education spokeswoman for the opposition Liberal Democrat party, alleged that the situation resulted from "years of Conservative neglect of our school buildings".

"Parents, teachers and pupils will be horrified that children have been taught in unsafe buildings and cannot return to school next week," she said.

"Instead pupils face more misery learning in temporary classrooms or being bussed miles to local schools."