The combustible insulation that was fitted to the outside of Grenfell Tower passed its official fire safety test after being partially covered by fireproof boarding that no one was told about.
A layer of magnesium oxide boards, which are marketed online as "completely non-flammable and used for fireproofing and as furnace liners," were added to some of the cement-based panels that Celotex claimed were the only product covering its combustible insulation.
Celotex RS5000 plastic foam insulation was then fitted to tower blocks without the magnesium oxide boards because Celotex did not publicise it had used them to pass the test and they were not mentioned in safety certificates based on BRE's reports of the materials tested.
The test was conducted at the headquarters of government fire safety advisors BRE which said it relied on Celotex to give them the correct information.
The admission from Celotex was made to the Grenfell Tower Inquiry, which is investigating the tragedy that killed 72 people in June 2017 when combustible insulation and combustible cladding panels helped to spread a fire that had started in a fourth floor flat.
It is not known who was aware that the test had not been properly conducted or reported in 2014, whether there was subsequently knowledge within the industry that the testing process was not robust, or if that could have had an influence on how strictly fire safety regulations for tower blocks were followed.
In April 2018, weeks after admitting its actions in correspondence to the Grenfell Tower Inquiry, Celotex paid to have the 2014 test conducted again without the fire-proof boards and passed.
In a statement on its website the firm said: "We commissioned a further test .. which aimed to mirror, as closely as possible, the system as described in the August 2014 report."
It went on: "The test results show that the rainscreen cladding system tested (which included RS5000) met the performance criteria."
The official test for combustible cladding for high rise buildings, known as British Standard 8414, is carried out on a nine metre high wall of insulation and covering boards built exactly as it would be on a real building.
Every constituent part, down to bolts and screws and foil tape, should be carefully recorded before a fire is lit underneath the whole cladding system and ultimately it is ruled to be either too dangerous for use or safe to attach to tower block flats.
The discrepancies in the 2014 RS5000 test were revealed in a report by fire engineer Dr Barbara Lane commissioned by the Grenfell Tower Inquiry.
In it she says Celotex fitted the magnesium oxide panels between the test fire and temperature sensors that determine whether the cladding passes or fails.
Dr Lane wrote that after the test fire was put out the cement-based panels "had fallen from the cladding rig".
But she said the magnesium oxide panels below the temperature sensors remained intact after the fire and the temperature recorded by the sensors above them gave the cladding system its coveted pass.
Dr Lane's report outlined other discrepancies between how Celotex said they had constructed the cladding system for the fire test and how they actually built it, and she says they also could have influenced the test result.
BRE, which invented the fire test for cladding that can burn and remains the only firm in Europe with the facilities to conduct it, told Sky News that Celotex didn't tell them about the magnesium oxide boards in the 2014 test and BRE's engineers and assessors didn't notice them.
"Regular observations are a standard part of the BS 8414 test but we don't have someone watching 24/7 or cameras documenting the installation," they told Sky News.
"Testing across every sector relies on the company testing to give the test house the right details on the product or system they are submitting – it is their responsibility to do so."
BRE also told us the combustible insulation and panel manufacturers who pay them to test their cladding are "entirely responsible for their product and related performance claims".
Celotex declined to say whether action had been taken against individuals involved in the 2014 test and said in its statement that it is "working to understand how those differences had arisen [and] this work is ongoing".
How and why the test was not conducted properly and any wider industry impact will be of interest to the Grenfell Tower Inquiry which is charged with investigating "industry practice relating to the design, construction, equipping and management of high-rise residential buildings".
The Metropolitan Police is conducting a separate criminal inquiry and is looking into 460 companies thought to have been involved in the refurbishment of Grenfell Tower.
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The disclosure that the only fire testing system for combustible cladding was not robust and was unable to identify its shortcomings until they were disclosed to the inquiry will also be of interest to the Department for Housing, Communities and Local Government.
The housing minister, Dominic Raab, has said combustible cladding will be banned from high rise blocks of flats in England, but his department has yet to announce exactly what products will be included in the ban or the height of buildings to which it will apply.