Hundreds of tower blocks in England could be covered in similar cladding to Glenfell Tower, councils have estimated.
So far tests have revealed that combustible cladding has been found on at least three tower blocks across the UK, the Government has said.
But Downing Street said English councils estimated that 600 high-rise buildings used similar cladding to the block in west London which was the scene of tragedy last week.
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.
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.