The government's announcement that schools needed to close because of safety fears came after concrete failed "with no warning".
Ministers have been under pressure to explain why they said schools would need to shut because of unsafe concrete just days before the start of the new term.
Earlier, schools minister Nick Gibb told Sky News "new evidence" emerged over the summer showing the dangers of reinforced autoclaved aerated concrete, known as RAAC.
Previously remediation was required when the RAAC was in critical condition, but Mr Gibb said the Department for Education (DfE) is now taking the "cautious approach" that all of the concrete should be removed.
He did not go into detail but later told BBC Radio Four's Today programme that a "beam collapsed" that had no external signs it was a "critical risk", prompting the change in guidance.
The DfE have since confirmed there have been a "small number of cases - including in education and non-education buildings - where RAAC has failed with no warning".
"Some of these cases are very recent," the DfE added.
Labour has accused the government of "neglect and incompetence" over their handling of the issue.
Mr Gibb admitted to Sky News on Friday that not all schools impacted by concrete safety fears have been contacted and it is not clear how many will have to shut fully.
He said in most cases "just a few buildings" or rooms within the affected schools will have to shut but "in some cases it will be the whole school".
Some schools yet to be contacted
Asked whether all affected schools have been contacted, Mr Gibb told Sky News: "The vast majority have, we've been calling them yesterday. But there is a few more that we're calling today."
However, asked for a number on the full closures, he said: "We don't know yet."
The government announced on Thursday that around 104 schools or "settings" in England found with concrete prone to collapse are set to be closed or disrupted - on top of 52 that have already been affected this year.
Labour is calling on ministers to "come clean" and publish the full list of schools that will be impacted, as they have not yet been publicly named.
Mr Gibb said the government intended to do that "in due course" but he wanted parents to be informed by the school before they read about it in the media.
He also suggested more schools could be affected as not all building surveys have been completed.
Concerns have been raised about RAAC, which is prone to collapse, for years.
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Pupils will be out of school 'for short period'
Mr Gibb insisted in cases where schools need to shut, children will only be out of face-to-face education for a "short period of time" - for an average of about six days, while the government fins alternative accommodation.
He said the costs of this will be covered by the DfE.
"We've made it very clear we will cover all capital costs," Mr Gibb said.
"So if in the worst-case scenario, we need portacabins in the school estate for an alternative accommodation, we will cover all those costs."
Government 'not being truthful'
Labour condemned the government for delay and inaction.
Shadow justice secretary Steve Reed told Sky News it was disingenuous to suggest the dangers of RAAC have only just come to light.
He told Sky News: "We know, and so do government ministers, that five years ago in 2018, there was a school in Gravesend in North Kent that collapsed because it had this kind of concrete.
"They had a report from the Department for Education itself just last December telling them the situation was critical at that point.
"In the last two years, my colleague Bridget Phillipson (shadow education secretary) has raised this issue in questions and debates in parliament over 150 times.
"So if they're telling you they didn't know this was a problem, they're not being truthful and they should have taken action the beginning of the summer holidays."