Every London council was warned by the fire service that cladding on high-rise buildings could be dangerous just weeks before the Grenfell Tower tragedy.
In a letter sent to all 33 local authorities and housing providers in the British capital in May, the London Fire Brigade (LFB) urged them to consider if panels could be flammable.
The safety advice came in the wake of a fire at Shepherd's Court in Hammersmith, west London, in August 2016, where cladding was found to have aided its spread.
The letter said: "In the case of this fire, we believe such panels were a contributory factor to the external fire spread."
Flammable cladding is suspected to have accelerated the scale of the west London blaze on June 14, which killed at least 79 people.
In the correspondence, a "number of cases" were said to be found where fire protection on external facades "did not comply" with building regulations.
The disclosure comes as British prime minister Theresa May confirmed 120 tower blocks have failed fire safety tests and face having their cladding removed.
Suggestions were made in the letter, signed by assistant commissioner Dan Daly, that contractors might have believed wrongly that separate safety certificates for glazing also extended to cladding.
The letter said: "In the light of fires that have occurred, I would urge you to consider carefully your arrangements for specifying, monitoring and approving all aspects of future replacement and improvement to building facades and construction of new buildings for which you are responsible.
"Contracts for the provision and installation of replacement elements of building facades, including insulation, replacement double glazing and associated spandrel and in-fill panels must ensure compliance with all parts of Part B if they are to secure public safety and minimise fire losses.
"I would therefore strongly urge that you consider this issue as part of the risk assessment process for premises under your control.
"I suggest that you make sure all relevant information about any replacement window and facade schemes is fully available to fire risk assessors.
"Where no reliable information is available for a given property, it is our general expectation that a strategy to assess the risk and where necessary implement short, medium and long term actions to address the risk.
"This assessment will need to take account of other fire safety measures already in place in the building as well as potential mitigation measures to ensure that any potential fire spread does not pose a risk to health and safety."
The Royal Borough of Kensington and Chelsea, where Grenfell Tower is located, would have received a copy of the letter, LFB said.
Kensington and Chelsea Council said: "We know there are many questions relating to fire safety standards.
"The council is committed to co-operating fully with both the public inquiry and the criminal investigation.
"We do not think it is right to make comments relevant to the inquiry or subject to the investigation until this issue has been discussed with the police and the solicitors to the public inquiry once they have been appointed.
"The council does not want to prejudice the fair conduct of the public inquiry in any way. We will update you as soon as those discussions have taken place."
