The former technical lead of a Leominster insulation maker whose product was used on Grenfell Tower has said he did not know that the product "burnt very ferociously" during a failed fire test.

Tony Millichap, Kingspan's head of technical between 2010 and 2015, told the public inquiry into the disaster he was also oblivious that the firm's Kooltherm K15 insulation was being sold with an unrepresentative fire test certificate.

The phenolic foam insulation, which was used in a small quantity on Grenfell Tower, is a popular insulation material for high-rise blocks.

But the inquiry has heard Kingspan, an insulation industry leader with a factory near Pembridge, changed the composition of the product after 2005 without amending marketing material. The outdated material stated that the insulation was suitable for use on buildings of more than 18 metres tall and had passed a relevant fire test.

The new version of the insulation – referred to as "new technology" versus "old technology" in proceedings – failed several cladding fire tests, the first in December 2007.

Technical team member Ivor Meredith noted in a report that the new technology "is very different in a fire situation to the previous technology" and had "burnt very ferociously".

However the inquiry has heard the firm kept selling it using the old technology's test pass from 2005 in the belief it would eventually pass the fire test.

On Wednesday, Mr Millichap told the inquiry he could not recall anyone telling him there had been internal concerns over the fire performance of the product they were selling to market.

He said he was under the impression that the 2005 test "absolutely" represented the product which was being sold.

Mr Millichap said: "I was aware of old technology being used in the 2005 test. What I didn't appreciate was that it didn't represent, or wasn't representative of, the product that was being sold laterally when I was in the role.

"I must admit the difference between old and new technology is lost on me.

"I worked on the assumption that the test which had been established and used in the business for over five years was representative of what was being supplied in the market.

"Knowing what I know now, that should have been looked into in more detail at the time, but I wasn't aware that could, should or did have an impact on its fire performance."

The 2005 test report was withdrawn by Kingspan only in October this year – a development Mr Millichap said came as a surprise to him.

Kingspan said: "It became apparent that the K15 manufactured in 2005 would not be representative of the product currently sold on the market from 2006 to today."

The firm has acknowledged "process shortcomings during the period of 2005 to 2014 for which it sincerely apologises".

However, it said building regulations at the time permitted K15's use on tall buildings providing the overall cladding system was compliant.

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

The proceedings continue.