Ministers must address a £8.5 billion backlog of repairs and replace “crumbling” schools with better designed buildings in order to ease overcrowding for pupils, the Royal Institute of British Architects has said.

In a report spelling out its demands for the next government, the RIBA warned more than three-quarters of schools still contained asbestos, and criticised the government’s replacement for Labour’s Building Schools for the Future programme.

The RIBA report said: “Apart from filling to the brim, British schools are also crumbling, creating poor learning and teaching conditions.

“Of the 29,000 schools in Britain, 80 per cent of the stock is beyond its shelf life, and a significant part of the school estate is in poor condition and insufficiently maintained.

“The estimated £8.5 billion backlog of repairs needed for existing schools are creating poor teaching and learning conditions and potentially exposing children and staff to health risks [due to asbestos].”

Architecture’s representative body also took aim at the government’s Priority School Building Programme, which it described as being too slow to address the needs of the most dilapidated school buildings.

““Progress has been slow - three years on from the announcement of PSBP, building work has started in fewer than 30 schools, with the first completed in May this year.”

“New school designs are 15 per cent smaller than those built under Labour’s Building Schools for the Future programme, with “smaller corridors, assembly halls, canteens and no standalone atria”, the report said.

Such designs can lead to “bullying, harassment” and overcrowding at peak times, it added.

The Department for Education defended its record on school buildings, stating the government was spending £18 billion on school buildings in this parliament.

“We are giving councils £5 billion to spend on new school places over this Parliament - double the amount allocated by the previous government over an equivalent period.”

“In addition we have also confirmed a further £2 billion for basic need up to 2017,” a DfE spokesperson added.