The GPMU has urged Dolphin Packaging in Poole to improve safety after an inquest into the death of a worker in September 1999.
Alan Callaway died at Salisbury District Hospital on 14 September, four days after receiving burns to 70% of his body in an explosion and fire at the plant.
The jury at Bournemouth Coroners Court returned an open verdict because the cause of the fireball remains a mystery.
A statement from Russell Jones and Walker, the Callaway familys solicitors, said: Managers at Dolphin Packaging were warned by safety inspectors about poor standards three times in the 20 months before the blaze.
The union decided to fund legal representation at the inquest for the Callaway family to highlight health and safety.
GPMU Wessex branch secretary Steve Attwill said: The inquest returned an open verdict, which reflects the fact that no proper investigation was possible as Dolphin failed to seal the area after the explosion. We believe that Mr Callaways tragic death could have been avoided if more stringent health and safety measures had been in place at the Dolphin works.
Dolphin Packaging recorded a turnover of 51m and pre-tax profits of 8.04m for the year to 31 December 1999. The firm refused to comment as PrintWeek went to press.
Story by John Davies
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