A director of the company that made combustible insulation used on Grenfell Tower has quit in the wake of evidence the firm used out of date fire tests to market material it knew could burn “like a raging inferno”. Peter Wilson will stand down in a fortnight as managing director of Kingspan’s insulation boards division and as a director of the plc. He is the only board member to leave the Irish company since the disaster on 14 June 2017, which killed 72 people. The move came as Gene Murtagh, the chief executive, told staff the company was trying to ensure “weaknesses in our testing and marketing of K15 can never be repeated”, admitting in an email “undeniable historic shortcomings”. Kingspan has an annual turnover of €3.5bn and its share price has fallen 15% in the last month. Wilson, 64, was in charge of the insulation division since 2001, during which time it launched Kooltherm K15, a plastic foam insulation used on the tower. Kingspan tested the foam in fire in 2005 and it passed, but then changed the chemical composition which made it more flammable, the public inquiry into the disaster heard. However, it continued to use the previous test pass and said it was safe for tall buildings. One of Wilson’s technical team threatened to “sue the arse off” a company that challenged its safety and another was tasked with running tests “to get the technology to pass, to justify our lie”. The inquiry also heard Wilson’s division lobbied certification bodies to give it approval, including not telling them about failed tests. The firm’s success inspired rival manufacturer Celotex to bring a similar combustible foam to market. It was this latter product that was mostly used at Grenfell. Lawyers for the bereaved and survivors have argued Kingspan, Celotex and Arconic, which made the plastic-filled cladding sheets, “were content to push hazardous products into the marketplace and sought to market them dishonestly”. Shortly before evidence about Kingspan was revealed at the public inquiry, triggering a share price fall, Wilson sold share options worth £1.6m, sparking anger among bereaved and survivors. Murtagh and Gilbert McCarthy, another director, also sold shares worth £3.1m and £1.8m respectively. The inquiry heard last week that after the 2017 disaster, the company continued to try to use testing to defend its position. In 2018 it set up tests and hired lobbyists in an attempt to persuade MPs that rival non-combustible products might be no less dangerous. It rigged the tests with deliberately “weak structural specification” and an assembly designed “to perform poorly”, according to internal emails. Kingspan executives will continue giving evidence when the inquiry resumes on 11 January.