Huws Gray has been fined £2.2 million after a worker was crushed to death by a 3 ton pallet of timber.
Paul Coulson was working in a conveyor at a Suffolk mill owned by the Llangefni-based building merchants in May 2024.
The Health and Safety Executive (HSE) said it had found CCTV evidence of dangerous working practices prior to the tragedy, but the company only made changes afterwards.
Mr Coulson (56), a labourer, was tasked with removing plastic packaging from timber pallets before they could be processed at Herringswell Sawmills in Bury St Edmonds.
He climbed inside the conveyor to access some of the packaging, but one of his colleagues started it without realising Mr Coulson was inside.
The operative, who could see that the pack of timber was not moving along the conveyor as it should, reversed it before changing its direction, which resulted in the pack of timber moving forwads again.
Mr Coulson, who was crushed twice by the pellet, died at the scene from his injuries.

HSE investigators found Huws Gray were aware that employees were accessing the danger zone within the conveyor and placed signage asking them not to do so - but CCTV revealed that workers had entered the conveyor 19 times between 14th April and 23rd May 2024.
Even though the company had placed stickers on the conveyor to tackle the practice, it did not take further action until after Mr Coulson's death.
These incluced guarding the conveyor to restrict access, changing work systems so that the pallets were unwrapped before being placed on the conveyor and more CCTV cameras to allow all angles to be seen by those operating it.
Although the company had placed stickers on the conveyor in a bid to tackle the working practice, no further action was taken to prevent access until after the tragedy.
The measures subsequently introduced to reduce the risk included guarding the conveyor to prevent access, changing the system of work so that the pallets were unwrapped before being placed on the conveyor, as well as more CCTV being installed to allow all angles to be seen by those operating it.
At Chelmsford Magistrates Court, Huws Gray Limited of Llangefni pleaded guilty to breaching the Health and Safety at Work Act and was fined £2.2 million. They must also pay full costs of £9,929.
HSE inspector Joanne Williams said: "This was a staggering failure that has cost a man his life and robbed a family of their loved one, and the scale of the fine handed down reflects the gravity of this case."
"All companies, regardless of their size, must follow the hierarchy of control for guarding dangerous parts within work equipment."
"Our investigation revealed that in this case, Huws Gray Ltd chose to control a serious risk through instruction alone – instead of putting proper safeguarding measures in place. This meant employees were able to access a danger area, with tragic consequences."
"Had Huws Gray Ltd taken robust action when they became aware of the problem, Paul Coulson would be alive today. Our thoughts will always remain with his family and friends for the loss they have suffered.”
An HSE spokesperson added that while health and safety standards are "much improved over recent years", sawmilling remains "a high risk industry" and recommends measures such as fixed distance guarding on the sides of conveyors and close guarding to meet the necessary reach distances.


Young man killed in A499 crash
Skipper in hospital after yacht breakdown
Anglesey's 'treasures' in new exhibition
Drug dealer jailed after £250k haul seized
Menai Bridge Aldi 'could create 40 jobs'