The cladding on the outside of the Grenfell Tower as well as the insulation used in the tower block both failed safety checks, the Metropolitan Police said, nine days after the blaze ripped through the building within minutes.
The force will consider manslaughter charges on those involved as part of its ongoing criminal investigation into the tragedy that claimed at least 79 lives and displaced hundreds.
The government has ordered immediate testing of the Hotpoint fridge freezer that was involved.
Whirlpool, which owns Hotpoint, said: "We offer our most profound condolences to the victims, those who have lost loved ones, homes, and possessions, and to their friends and families."
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The police report confirms the fire had not been started deliberately and manufacturers of the fridge-freezer which triggered the fire, Hotpoint, have been contacted.
The specificFF175BP model that burst was not found to be on any previous product recall list or classified as dangerous in any way.
"Preliminary tests show the insulation samples collected from Grenfell Tower combusted soon after the tests started. The initial tests on equivalent aluminium composite tiles failed also," said Met Police Detective Superintendent Fiona McCormack, who is overseeing the investigation.
She said the insulation proved "more flammable than the cladding" and investigators will now seek to establish whether the use of any of these materials was illegal in the UK.
Documents and materials have been seized from a number of organisations as part of this process, the police said.
The blaze on June 14 had destroyed 150 homes in the north Kensington tower block and has since triggered safety checks on hundreds of tower blocks around the UK.
As many as 11 residential high-rise buildings in eight local authority areas of the country have been found with cladding which raises safety concerns.
The UK's Department for Communities and Local Government has written to all local authority and housing association chief executives to advise them on steps to take if tower blocks in their area are found to be covered in cladding they are concerned about.
The letter says that the Army will be assisting with building repairs.
Cladding is typically fitted to the outside of high-rise buildings to improve insulation and tidy up the appearance of the exterior.
A Downing Street spokesperson said no-one would be left to live in unsafe buildings with cladding found to be flammable.
"They will be rehoused if they need to be and landlords will be asked to provide alternative accommodation where that's possible," the spokesperson said.
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