The cladding on the outside of the building as well as the insulation used in the tower block both failed safety checks, the Metropolitan Police said.
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.
"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.
Documents and materials have been seized from a number of organisations as part of this process, the police said.
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"We are looking at every criminal offence from manslaughter onwards, we are looking at every health and safety and fire safety offences and we are reviewing every company at the moment involved in the building and refurbishment of Grenfell Tower," McCormack said.
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 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.