The insulation that was fitted on Grenfell Tower never passed the required safety tests and should never have been installed on the building, a BBC investigation has claimed.
Celotex, the manufacturer of the RS5000 insulation, used extra fire retardant in the product so that it qualified for a safety certificate, the Panorama programme has reported.
The programme believes a more flammable version of the product was then sold for public use, and the plastic foam insulation was used on hundreds of other buildings, including the refurbishment of Grenfell.
The RS5000 insulation gives off toxic fumes, which contain cyanide, when it burns, Panorama reported.
The accusations come as a public inquiry into the blaze that killed 72 people in west London last year opened on Monday.
Panorama also alleges the tests showed RS5000 was only safe to use on certain new build projects when it was combined with a specific fire-proof cladding panel.
The programme says its reporters found Celotex knowingly misled buyers as its marketing suggested the insulation was suitable for use with other cladding panels and for tower block refurbishment projects, which was not the case.
Panorama says it discovered Celotex targetted contractors who were refurbishing Grenfell Tower, and specifically offered its flammable insulation despite the company knowing it was going to be combined with combustible cladding panels.
Celotex told Panorama it could not comment because it is co-operating with the police investigation and the public inquiry but did not deny the allegations.
Fire safety expert Arnold Tarling said he was shocked by the revelation about RS5000 insulation. “Words fail me,” he said. “This is absolutely mind-blowing. This material is all over the place.”
Matt Wrack, general secretary of the Fire Brigades Union, said the allegations should be investigated. “If there are breaches of the law then those people need to be held to account,” he said.
Celotex said: “We believe that the right forum for considering and assessing the many, complex and inter-related issues which arise in relation to the fire – and which require consideration of the involvement of all relevant parties – is through these official investigations.
“We do not think it is appropriate to comment any further outside of or in advance of that process.”
When Panorama told Celotex that its actions might amount to corporate manslaughter, the company said: “We fully recognise the seriousness of the Grenfell fire.
“It is for this reason that we believe the public inquiry and the police investigation are the right processes to consider the events leading up to the fire, and the night of the fire itself.”
Robert Bond, chief executive of the main contractor Rydon, told the programme that testing of the cladding system wasn’t required because “it was deemed to comply”.