Tests on tower blocks following the Grenfell Tower fire has shown cladding on "a number" are "combustible," the Prime Minister has said.
Theresa May said local authorities and fire services concerned are taking "all possible steps" to ensure the buildings are safe and residents have been informed.
She said the Government had arranged to test cladding on all relevant tower blocks as a precaution.
"A number of these tests have come back as combustible," she told the House of Commons.
"The relevant local authorities and local fire services have been informed and as I speak they are taking all possible steps to ensure buildings are safe and to inform affected residents."
Harriet Harman, a Labour MP, described the revelation as "chilling".
Ms May went on to urge any landlords who own tower blocks to send samples for testing as soon as possible.
She said any results will be "communicated immediately to local authorities and local fire services."
The Prime Minister said: "Land lords have a legal obligation to provide safe buildings and where they cannot do that we expect alternative accommodation to be provided.
"We cannot and will not ask people to live in unsafe homes."
Ms May, making a statement to the Commons, said "no stone will be left unturned" in the inquiry.
She also said: "For any guilty parties there will be nowhere to hide."
Ms May said she expects an interim report to be produced as "early as possible" by the chair of the inquiry.
The Prime Minister also said the tragedy will not be used to carry out immigration checks on those affected by the tragedy or on those providing "vital" information to identify victims or to assist the criminal investigation.
The Labour leader, Jeremy Corbyn, said the residents of Grenfell Tower were "let down both in the immediate aftermath and so cruelly beforehand".
He said the public inquiry "must establish the extent and by who".
"At least 79 people are dead - it is both a tragedy and an outrage because every single one of those deaths could and should have been avoided," he added.
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