Jonathan Roper, the ex-assistant product manager at Celotex, admitted to the Grenfell inquiry that the firm had committed a “fraud on the market” by “overengineering” a fire safety test to pass and then wrongly marketing the material as being safe.

He revealed that after a failed fire test in January 2014, Celotex added a non-combustible magnesium oxide board to the RS5000 insulation test panel system to prevent it from failing another fire test three month later.

The young former business studies graduate told the inquiry his superiors ordered the removal of any mention of the magnesium oxide in marketing literature, which he agreed was “misleading and intended to mislead”.

The firm then went on to claim the combustible rigid foam boards were safe for use on high-rise buildings

He told the inquiry: “All of the members of the management action board I believed were present when … presented [with] the truth and the decision was made.

“Subsequently I’ve heard and seen that this isn’t the only manipulation of test data that the business has had.

“I think there are issues around the class 0 fire report, I think there are issues around the thermal performance of their entire product range. It was clearly within the culture of that business at that time, and I’m sorry for my part in it.

“I went along with a lot of actions at Celotex that,  looking back on reflection, were completely unethical, and one that I probably didn’t potentially consider the impact of at the time. I was, as we’ve said, I was 22, 23, first job. I thought this was standard practice.”

The proceedings continue.