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.