More from Education Secretary for England, Gillian Keegan, on BBC Breakfast.

"The vast majority of surveys come back without RAAC.

"But where we find it we will then treat all of them as critical and we will prop them up or put temporary accomodation into place," she says.

Keegan tells the programme there have been reports there could be thousands more cases, but she insists that will not be the case.

"It could be hundreds, but we need to get the surveys back. We started in March last year," says Keegan - adding they still have outstanding surveys from schools across England.

Education Secretary Gillian Keegan confirms the number of schools in England affected - needing either full or partial closure - is 104.

"These were all the ones that were identified out of the 15,000 built in the period [when RAAC was used}.

Only 1% of surveyed schools had any RAAC, she tells BBC Breakfast.

The Department for Education categorised their safety concerns as being either critical or non-critical, says Keegan.

"The critical 52 were already done. The 104 were deemed as non-critical - the advice and guidance was to monitor those and they didn't need any particular action."

But when the guidance has changed, there are now 104 which are deemed to be critical.

"We need to get them done," says Keegan.

Education Secretary Gillian Keegan is on BBC Breakfast now, speaking about how the government is responding to the RAAC issue in schools across England.

Keegan says there are several companies contracted by the government to assist schools.

"Normally we wouldn't do this, the responsible bodies would do this, but to make it much more efficient we've centrally taken that on board so we can a) pay for it and b) make sure that it's very quickly available," Keegan says.

Keegan says 104 schools are not mitigated as of this morning - as in they have not put measures in place to ensure the safety of pupils.

Over the summer, engineers were sent to two incidents, she says, in one a roof panel made with RAAC had collapsed, having previously not been deemed a critical safety issue.

This, Keegan says, triggered the urgent safety announcements of last week.

Jonathan Slater, who was the top civil servant at the Department for Education from May 2016 to August 2020, tells the Today programme a survey of schools was carried out about 10 years ago and a second was commissioned in his time. These showed the the scale of the rebuilding programme needed.

According to that, 300 to 400 schools needed to be repaired per year "because of the system the schools were built with was post-war concrete and it was only supposed to live for 30-40 years".

When they went to the Treasury, they got funding to rebuild 100 schools per year.

"It was frustrating when your priority was safety," he says.

Slater says the education department saw the benefits of the program but "the challenge was to face the chancellor".

In 2021, they wanted to be realistic and asked the government to double the 100 schools to 200.

"We know 300 to 400 needed, but the actual ask in 2021 was to double the 100 to 200. I thought we'd get it but the actual decision made in 2021 was to halve down from 100 a year to 50 year," he adds.

And he reminds the programme that the chancellor then was Rishi Sunak.

Peter Smith, head teacher of Farlingaye High School in Woodbridge, Suffolk, says around a third of his classrooms, as well as toilets, changing rooms and office spaces have RAAC concrete.

Speaking to the Today programme on BBC Radio 4, Smith says RAAC was identified during roofing work in May.

The school was not initially contacted over RAAC safety fears, Smith says. Instead he wrote to the Department for Education on Thursday last week, and on Friday received the confirmation those spaces were unsafe.

"This can't be another Covid scenario where school leaders found out about things at the same time as everyone else," Smith says.

"This is a crisis for schools. We need the reassurance that this is going to be all hands to the pump."

Thangam Debbonaire, shadow leader of the House of Commons, says the government should come clean about the scale of the RAAC concrete issue.

Speaking to BBC Radio 4's Today programme, Debbonaire says "One thing we know is that parents, teachers and staff are wondering if their school building is safe."

She says that "today is much too late but it's better than never".

What would her party do differently, Debonnaire is asked?

Labour, she says, would first start with publishing the list of schools "because we have no idea how much this would cost."

When asked how much money will be allocated to rebuilding schools if the Labour wins, she says: "We don't know what money we are going to have in public finances.

"We could not make a commitment right now until we know the scale of the problem."

Image source, UK Parliament

So as pupils wake up, ready to start the new academic year in much of the UK, some will find their school is either partially fully closed due to the presence of potentially unsafe RAAC concrete in their buildings.

For 104 schools in England where RAAC has been identified, dozens are delaying their re-opening or asking some pupils to learn remotely, writes our education editor Branwen Jeffreys.

The government says it hopes to survey the remaining schools which need to be looked at within a few weeks. As more buildings are checked the number taken out of use may increase.

While RAAC is the most pressing issue there are wider questions about the safety of some school buildings because of asbestos and older prefabrication methods.

Documents seen by the BBC show Downing Street was warned as early as March last year that some school sites posed a “risk to life”. Spending on school buildings in England has fallen sharply over the last decade in real terms

Niamh Sweeney, deputy general secretary of the National Education Union, says she doesn't believe the RAAC concrete issue can be solved in days, rather than weeks.

"We have seen since June that affected buildings have had propping up mitigations put in place. That is still ongoing," she says.

Speaking to BBC Breakfast, Sweeney adds that some hospital buildings have been dealing with the issue since 2021.

"We need confirmation from the government that this will be dealt with swiftly," she believes.

She also says parents should be reassured that head teachers will have made inspections and further investigation.

According to Sweeney, the Department of Education put pressure on head teachers by saying a site inspection is good enough.

"What we are saying is that the government needs to take responsibility, they need accurate records of that all of those inspections have been taking place," she concludes.

Speaking to the BBC last week, some parents described their shock at being told their children's schools need to close due to the risk of dangerous concrete collapses.

Some said the short notice left them scrambling to arrange childcare.

"This is an absolute disgrace," said Wendy Kirwood, who was told several corridors, a library and sports hall at her son's school in Workington, Cumbria, were affected.

Martina Eliasova's daughter had been due to start Year Two at Katherines Primary Academy in Harlow, but the start of term has been postponed to 11 September.

"I don't have family here. I can't say 'mum, can you help my daughter?' I have to either take holiday, or just have her home and somehow battle through," she said.

Pascal Dowling, whose child's school in Somerset has been affected, said she wanted the government to "bear responsibility for yet another catastrophic failure".

"Our children have already had two years of their education turned upside down by Covid countermeasures and now face a winter trying to catch up in hastily constructed sheds," she said.

Read more on the reaction from parents here.

As we mentioned earlier, it’s not just schools that have been built with the lightweight concrete at risk of collapse.

​​Hospitals, courts, police stations and other public buildings have also been built with reinforced autoclaved aerated concrete (RAAC) - a popular choice for roofs, floors and walls between the 1950s and 1990s.

​​RAAC planks are thought to be present in 26 hospitals, while six court buildings are affected, according to government sources.

​​The government says it is yet to draw up a definitive list of its own buildings affected by RAAC.

​​So, as with the RAAC crisis in schools, the scale of the issue across the public sector is relatively unknown.

You will likely have heard the word RAAC (pronounced: rack) a lot over the past few days. But what actually is it?

​​Reinforced autoclaved aerated concrete (RAAC) is a lightweight material that was used mostly in flat roofing - but also in floors and walls - between the 1950s and 1990s.

​​It is aerated, or "bubbly", and a cheaper alternative to standard concrete, and is quicker to produce and easier to install.

​​But it is less durable and has a lifespan of around 30 years.

​​It can also deteriorate when exposed to moisture.

​​The bubbles can allow water to enter the material, and if that happens, any steel bar reinforcing the RAAC can also decay, rust and weaken.

So what exactly did the Chancellor say on Laura Kuenssberg's Sunday programme?

​​When asked if he would commit to covering "whatever costs" were required to remove dangerous concrete from schools, he said: "We will spend what it takes to make sure children can go to school safely, yes."

​​He didn't give an exact figure on what those costs would be, and the Treasury has since said that promise won't mean any new money outside of current budgets.

​​But he has committed to doing" absolutely what it takes" to address the issue.

The return of pupils to classrooms after the summer holidays is being disrupted for more than 100 schools across the country.

​​Pressure is being piled on the government to share a list of schools forced to close classrooms days before the start of the new term due to the risk of dangerous concrete collapses.

​​More than 100 schools in England have been told to close buildings where reinforced autoclaved aerated concrete (RAAC) is present without safety measures in place.

​​But it’s not just schools. We know now that other public buildings - including courts and hospitals - were built with the material. But, again, there is no definitive list of affected government premises.

​​Over the weekend, Chancellor Jeremy Hunt told the BBC that the government would spend “what it takes” to fix the RAAC crisis in schools, but he refused to be drawn on the likely cost.

​​All told, questions remain over the scale of the problem.

​​Stay with us as we bring you the latest news, reaction and analysis.