Hundreds of tower blocks in England have similar cladding to that used in the Grenfell Tower fire disaster, Downing Street has admitted.
So far tests have revealed that combustible cladding has been found on at least three tower blocks across the UK, the Government said.
But English councils have estimated that 600 high-rise buildings used cladding similar to Grenfell Tower in west London, where at least 78 people died last week after the building was engulfed in flames.
Councils were told to provide the Government with details of the cladding used on high-rises by Monday and three samples were found to be combustible after tests on a "small number" of specimens.
Flammable panelling on the outside of Grenfell Tower is suspected to have aided the rapid spread of last week's blaze, trapping dozens inside.
Speaking in the Commons earlier, the Prime Minister, 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.
Ms May went on to urge any landlords who own tower blocks to send samples for testing as soon as possible.
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."
Harriet Harman, a Labour MP, described the revelation as "chilling".
The Department for Communities and Local Government is coordinating the process and facilities allow for 100 samples a day to be tested.
A Number 10 spokeswoman said: "So far, three samples have been found to be combustible."
She added: "In terms of how many buildings and how many homes have this type of cladding, the estimate provided to us by councils is that there are approximately 600 high-rise buildings with similar cladding.
"We are in touch with all the local authorities to encourage them to urgently send us the samples and then we will carry out the checks that we need to see where we are with that."
In blocks where the cladding is found to be combustible "we will do a further test to make sure the building is safe" and residents could be rehomed.
"Obviously nobody will be living in buildings that are unsafe, they will be rehoused if they need to be and landlords will be asked to provide alternative accommodation where that's possible," the spokeswoman said.
