A worker for Suffolk-based Celotex wrote “WTF” on a diagram of its flammable insulation used on Grenfell Tower – after learning the product’s fire test had been manipulated, an inquiry heard.

Debbie Berger told the inquiry into the June 2017 tragedy that she annotated the diagram of the Rs5000 product on a test rig with the acronym as an “expression of shock”.

The inquiry into the fire, which killed 72 people, has previously heard how a test of Rs5000 was “overengineered” to get a pass in a few short months after a first test failed.

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Celotex, based in Lady Lane, Hadleigh, added a 6mm fire-resisting magnesium oxide board to a cladding test rig and 8mm fibre cement panels were added over the magnesium oxide to “conceal” its presence.

After a first test failure in January 2014, the second system which passed in May 2014 was used to erroneously market the combustible rigid foam boards as being safe for use on high-rise buildings such as Grenfell Tower.

Ms Berger became product manager in 2014, just months before the test was passed.

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The inquiry was told Ms Berger had just realised additional materials had been added to the insulation to get it to pass a safety test for use in high-rise buildings when she made the “WTF” note.

“It’s shock, shorthand for shock that this could have happened,” she said during evidence on Thursday.

When pushed by inquiry lawyer Richard Millet QC as to why she did not raise the issue despite her shock, Ms Berger said it “receded” from her mind as she took on a lot of information for her new role.

She added: “It really didn’t imprint itself. There was a lot of technical information that I was being shown at the time.

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“I remember talking about this and there was a lot of technical information, and some of it stuck, some of it didn’t.”

She denied deliberately hiding the manipulation - which has been the focus of questioning for Celotex employees at a number of recent inquiry hearings.

She said: “If I had recalled it and known about it, I would have shared it with the management of Celotex because I shared other details, some of which weren’t talked about, I was honest about those.”

Suggesting she did not fully understand the detail, Ms Berger added: “I suppose somewhere in the back of my mind I thought ‘it’s passed a test’.”

She said of her former employer: “I didn’t think Celotex would do this, I thought Celotex was a good company, I thought Celotex prided itself on doing the right thing and being honest, and I was really shocked by this.”