Workers at the Salford plant are also planning to take industrial action consisting of a continued over-time ban.

Members have taken action after a breakdown in negotiations with the company over a long running disagreement regarding a pay freeze agreement.

According to the union, API Holographics, part of the API Group, is refusing to offer an "acceptable review" to existing staff under a pay neutral agreement, signed in 2009.

Employees agreed to the freeze when the company was in "some financial difficulties and making redundancies at the site", the union claimed.

It said that the company's fortunes had now turned around, which should trigger a clause in the agreement, which should see the pay freeze scrapped.

Unite regional officer Alison Jones said: "Our members in Salford feel they have no choice but to take this action as discussions with the company have failed to provide a satisfactory outcome.

"The staff have always worked with their employer to ensure that it remains competitive and have shown their willingness to compromise regarding previous wage claims. However, as the company has now returned to profit, they just wish to be rewarded for the commitment and support they have given as these workers have not had a pay rise since 2007.

"We remain available to talk with the company at any time to resolve this issue and urge management to return to the negotiating table."

Workers have also given notice to stage further 24-hour stoppages should the dispute remain unresolved.

In a statement to PrintWeek, API Holographics managing director Stephen Clarke said: "We are disappointed by the action being taken by union members. Two years ago, at a time when API Holographics was in a very difficult trading position, the company offered a four year pay freeze because we believed the alternative of a wage cut would be detrimental to employees. This was voted on and accepted by the workforce.

"This agreement allowed the company to plan its return to profitability and growth supported by stable wage costs . It is disappointing that only two years into the agreement, just as the strategy is starting to bear fruits and allow us to consider further investment, some employees have decided to take this action.

"Despite claims made, there is certainly no clause in the agreement that a return to profit makes it invalid. The company is simply asking all those involved to honour the existing agreement. We recognise the vital part employees have made in the improvements already achieved and have offered several alternatives but these have been rejected.

"In the meantime, we are taking all measures to minimise the effect of this dispute on customers by ensuring contingency plans are in place. This action does not affect other API operations in the UK or abroad.

Updated to include API statement.