Senior staff at an insulation firm knew the fire certificate for a product used on Grenfell Tower was misleading for four years before they informed the public, an inquiry has heard.

The head of marketing at Kingspan did not think there was "any difference in the fire performance" when their K15 product was being sold using test data which was almost 15 years old.

Adrian Pargeter, who started working for the company in 2009, also blamed preparations for the inquiry and Covid for a further nine-month delay between Kingspan confirming with documentation that their tests did not match the marketing material, and writing to the industry to tell them.

Kingspan sold its Kooltherm K15 insulation with an unrepresentative fire certificate based on a 2005 test which claimed it was safe for use on buildings higher than 18m.

The product sold from 2006 onwards was different from that tested the year before, but the firm kept selling it using the old technology's test pass and only withdrew this certification in October.

Head of technical and marketing at the firm, Mr Pargeter said in his statement to the inquiry: "I think I first became aware that the 2005 BS 8414 test may have used a different version of K15 than sold post-2006 towards the end of 2016.

"My detailed understanding of the issue started to develop from the end of 2018 onwards."

He added: "I was not concerned about the issue at this time and did not investigate it further given that [Kingspan] had been relying on the test for years and I was not aware of any indication that the change had any impact on the product's fire performance."

He said the issue arose again two years later in 2018, when the Grenfell Inquiry got in touch to collect evidence for the hearings.

Kingspan wrote to the inquiry and industry bodies in October this year to inform them that the test certificates for Kooltherm K15 had been formally withdrawn.

Mr Pargeter suggested the team at Kingspan knew the product tested was different from that being sold by early 2019, however it was a further 20 months before this was reported to the inquiry and industry following a re-test in June last year.

He explained: "I think we'd come to the conclusion that it wasn't sufficiently representative of the product sold, that would have been in February or March 2019, and then we undertook the testing programme, which went on then and we had the reports then in February 2020."

When asked about the discrepancy between them receiving the reports and not writing the letter until eight months later, Mr Pargeter said: "I think because we were just so busy we didn't really think about writing out to the engineers again until August September and then we started putting this letter together then.

"It was just busy, answering questions to the inquiry and running the business during Covid times.

"It was an oversight."

The technical head also told lawyers that Kingspan were "more focused" on K15 by the time they withdrew certificates than they had been in 2016.

He told inquiry lawyer Richard Millett QC: " I think we were looking at it in a different light, much more focused on it."

He added: "The opinion then was it would be prudent to withdraw it at this stage."

Mr Millett suggested the only thing that was different was the degree of scrutiny Kingspan and the K15 product were facing as a result of their inquiry questioning.

Mr Pargeter agreed that was "potentially" the case.

The firm has said it did not provide any advice about the suitability of K15 for use on Grenfell Tower and that the firm only learned a small amount of the insulation had been used on the building after the June 2017 fire, which killed 72 people.

The majority of the flammable insulation used on the tower was made by rival firm Celotex.

The inquiry continues.