Rishi Sunak refused to fully fund a programme to rebuild England’s crumbling schools, a former senior official at the Department for Education has claimed.

Thousands of pupils face disruption at the start of term this week following an order to fully or partially close 104 schools because of concerns about collapse-prone reinforced autoclaved aerated concrete (Raac).

But Jonathan Slater, who was permanent secretary at the Department for Education from May 2016 to August 2020, claimed the Treasury had failed to fully fund school rebuilding schemes – including while Mr Sunak was chancellor.

He said he was “absolutely amazed” that a decision was made after he left the department to halve the school rebuilding programme.

Mr Slater said up to 400 schools a year need to be replaced, but the DfE got funding for 100 while he was the senior official, which was “frustrating”.

But when he left the department he said he was “optimistic” that a push for extra funding would be successful.

He told BBC Radio 4’s Today programme: “The actual ask in the Spending Review of 2021 was to double the 100 to 200 – that’s what we thought was going to be practical at first instance.

“I thought we’d get it, but the actual decision that the chancellor took in 2021 was to halve the size of the programme.”

The issue of England’s ageing schools – highlighted by a National Audit Office report which said up to 700,000 children are being taught in buildings that need replacing or major refurbishment – has become a political storm.

Just days before the start of term more than 100 schools in England were told to fully or partially close as a result of safety concerns about Raac, which was widely used from the 1950s to mid-1990s.

The Department for Education has so far refused to say which schools are affected.

Education Secretary Gillian Keegan has vowed to publish a list of the schools affected by the concrete crisis this week.

She told BBC Radio 4’s Today programme: “We will publish the list, but I do want to double-check that the school has had the opportunity – because not all the schools are back yet – to tell all parents.”

She said three companies providing portable buildings have already been contracted to set up temporary classrooms.

“Many schools are either looking for alternative accommodation, if they’re within a multi-academy trust or within a local authority, or moving to another classroom if they’ve got spare classrooms,” she told Sky News.

“If it’s across the whole school, then that gets more difficult. So what we’re doing right now is we’ve assigned a caseworker for each one of the schools, working with the school to figure out what the mitigation plans are.”

She defended the Tories’ record on school funding, in response to Mr Slater’s comments.

“I’ve just announced 239 school rebuilding projects,” she said, but could not indicate how many a year, saying only that they would be done “as soon as possible”.

She added: “We’ve delivered much better value for money, much more schools have been rebuilt, much more schools are going to be rebuilt, we’ve got a grip of Raac.”

Shadow education secretary Bridget Phillipson said: “The defining image of 13 years of the Conservative-run education system will be children sat under steel girders to stop the roof falling in.”

She said Mr Sunak “bears huge culpability for his role in this debacle”, adding: “Ministers need to come clean about the number of schools affected, what they knew, and when they knew, about the risks posed by Raac so that parents can be reassured their children are safe at school.”

Liberal Democrat education spokeswoman Munira Wilson said: “This bombshell revelation shows the blame for this concrete crisis lies firmly at Rishi Sunak’s door.

“He slashed funding to repair crumbling classrooms when officials said it needed to be increased. Now children and parents across the country are paying the price for this disastrously short-sighted decision.”

Schools in Scotland, Wales and Northern Ireland are also being assessed for Raac.

The Scottish Government has said it is present in 35 schools, but that none poses an “immediate risk” to pupil safety.

The Welsh Government said councils and colleges have not reported any presence of Raac.