The Fire Brigades Union has criticised cladding manufacturers on the basis of Grenfell inquiry evidence for a “total disregard for safety in the pursuit of profit” that the trade union describes as “stomach-churning”.

The inquiry currently looking into the Grenfell Tower fire that caused 72 deaths in 2017 has been told that Celotex, the main supplier of insulation for the building, cheated in order to pass a fire safety test in 2014.

Former Celotex assistant product manager Jonathan Roper admitted on Monday that the company behaved in a “completely unethical way” by “over-engineering” a cladding test, amounting to “dishonest” work.

After the first phase of the inquiry concluded that the flammable cladding on the west London tower fuelled the fire, the second phase now underway is examining the circumstances that allowed the tragedy to take place.

It has been revealed via evidence to the inquiry that Celotex, after failing a first test, added a fire-resisting magnesium oxide board to a cladding test rig made up of 12mm fibre cement panels for a second test.

Any mention in marketing literature of the magnesium oxide was then removed by order of superiors in the company, according to former employee Roper, which he described as “misleading and intended to mislead”.

The inquiry today heard that Celotex exploited ignorance about fire safety classifications in local authority building control to wrongfully obtain a certificate, which allowed their product to be used on buildings over 18 metres.

FBU general secretary Matt Wrack said: “Today, we’ve seen further evidence that Celotex deliberately misled buyers and regulators into thinking their highly flammable insulation was safe for high-rise buildings.

“The total disregard for safety in the pursuit of profit is stomach-churning – and it’s a symptom of the shamefully lax regulatory environment fostered by consecutive governments.

“Each day of phase two the Grenfell Tower Inquiry is more shocking than the last. But sadly the interest from many media outlets has waned, now that corporations are on the stand.”

Labour MP Zarah Sultana tweeted: “The Grenfell Fire Inquiry has been told that Celotex, who supplied insulation for the tower’s refurbishment, deliberately cheated to pass a fire-safety test in 2014. This is an absolutely huge scandal.”

Her colleague David Lammy, a shadow cabinet member who personally knew one of the Grenfell victims, Khadija Saye, said: “This is totally scandalous and should be on every newspaper’s front page.”

Wrack, head of the union that represents the overwhelming majority of firefighters in the UK and is a core participant in the Grenfell inquiry, concluded: “What happened at Grenfell was criminal – and those responsible should be properly held to account.”

The FBU highlighted in September that emails seen by the inquiry revealed “just how lax” the government allowed UK building regulations to become and said ministers “showed no interest in tackling the problem”.