The Bootle site, part of US-owned packaging group Crown Holdings, employs around 150 staff and produces metal cans and packaging for UK firms such as Cadbury, Jacobs and Jeyes.

Around 28 employees, who make up the firm’s print unit, and a further eight from other divisions, are taking strike action after a 7% pay increase for all print staff that was agreed and confirmed in writing by management last year failed to materialise.

The deal, which also involved a new seasonal shift pattern, was due to coincide with the arrival of a new four-colour printing press. However following its recent installation staff were told that only those directly responsible for operating the press – around eight – would be entitled to the rise, according to Unite regional officer Phil Morgan.

He said: "The increase was clearly part of the agreement, there was no ambiguity at all. We received it in writing and they have now come back at the point the press is being commissioned saying it will only apply to the operators."

Union members have observed an overtime ban for the past month and took part in the first of three planned stoppage days, on Friday 28 June. A further two are planned for tomorrow (5 July) and next Friday (12 July).

Morgan said: "We haven’t heard anything from them at all since this started. We are open to negotiation but they haven’t indicated to us that they are open to that. In our final meeting when we indicated this was the likely route we would go down, it didn’t change anything."

No-one was available to comment at the company.