Unite members began walking out for four hours every Friday on 6 May after the company refused to discuss a pay rise, following a 13% pay reduction 18 months ago.

Legislation dictates that employees can only strike for 12 weeks, at which point a second ballot has to be taken.

According to Unite, members voted to continue action in the same vein, which also involves all breaks being taken in their entirety. However, national officer Steve Sibbald said that the union hadn't ruled out stepping up the action.

He said: "It was always going to go one way or another - either the members would stop all together or vote overwhelmingly to continue the action.

"I think the company probably hoped that the second ballot would have gone the other way, but the members' resolve seems to harden every week. I don't know how long they will carry this on, but the members don't look like they are ready to stop."