Further schools could be told they need to shut classrooms because they are fitted with a concrete that could suddenly collapse, the schools minister has admitted.
Nick Gibb insisted that the Government will pay for the costs of temporary accommodation after official guidance suggested schools will have to cover the emergency measures.
The Department for Education (DfE) has told 104 schools and colleges to partially or fully close buildings just as pupils prepared to return after the summer holidays.
But Mr Gibb conceded that more schools could be told to make closures as evidence-gathering continues over the presence of reinforced autoclaved aerated concrete (Raac).
"There may be more after that as these questionnaires continue to be surveyed and we continue to do more surveying work," he told GB News.
But Mr Gibb insisted that pupils and parents should not be apprehensive about the risk during the wait for the results.
"No, they shouldn't worry," he said.
"That's a very cautious approach, so parents can be confident that if they've not been contacted by their school it is safe to send children back into school."
He insisted "we took the decision as soon as the evidence emerged" as ministers faced anger for only telling schools of the closures days before children start the autumn term.
Mr Gibb said "over the summer" they discovered a number of instances where Raac that had been considered to be low risk "actually turned out to be unsafe".
He told BBC Radio 4's Today programme that a "beam collapsed" that had no external signs it was a "critical risk".
The minister said the official guidance was being altered after it triggered additional anger by suggesting schools will have to pay for rental costs for emergency accommodation.
"We're going to clarify the guidance because of their misinterpretation, we are paying for those costs," he told Today.
"If in the worst-case scenario a school does have to close and we put Portakabins into the grounds, all that cost will be covered by the department."
On Thursday, the DfE said it had contacted the 104 more schools after 52 of the 156 educational settings containing the concrete took protective steps so far this year.
The department said a "minority" will need to "either fully or partially relocate" to alternative accommodation while safety measures are installed.
Space in nearby schools, community centres or in an "empty local office building" was recommended for the "first few weeks" while buildings are secured with structural supports.
Schools were told moving to pandemic-style remote education should only be considered as a "last resort and for a short period".
Labour has demanded that the Government names all the affected schools.
Mr Gibb insisted schools were contacting affected families and told Today "we will publish a list", but only once they are in a "stable place".
Raac is a lightweight building material used from the 1950s up to the mid-1990s, but is now assessed to be at risk of collapse.
The DfE has been considering Raac as a potential issue since late 2018 but the timing of the decision to issue guidance just days before the start of term has angered unions.
National Education Union general secretary Daniel Kebede said: "It is absolutely disgraceful, and a sign of gross Government incompetence, that a few days before the start of term 104 schools are finding out that some or all of their buildings are unsafe and cannot be used.
"To add insult to injury the Government states in its guidance that it will not be covering the costs of emergency temporary accommodation or additional transport."
The local authority in Bradford revealed on Thursday that Raac was detected in Crossflatts Primary School and Eldwick Primary School in the West Yorkshire city.
Both interim and long-term alteration works are being carried out to ensure children can be accommodated on the two sites, according to Bradford Council.
The council said interim alterations to safe areas will be finished by Sunday and temporary classrooms on both school sites have been ordered and should arrive within the next 8-10 weeks at Crossflatts and 14-16 weeks at Eldwick.
Elsewhere, the BBC reports affected schools include Ferryhill School, a secondary in County Durham, Willowbrook Mead Primary Academy in Leicester and Corpus Christi Catholic Primary School in Brixton, south London.
