In a ballot carried out last month, members opted to take industrial action short of a strike, in response to a nationwide pay freeze last November.

Union members have opted to "work to rule", which will include working to exact hours and exactly following guidelines set out by the company.

The action is expected to begin on 19 September and Unite national officer Steve Sibbald said: "We will continue indefinitely until we get a response from the company."

However, the two parties will continue with negotiations in the hope of thrashing out a solution before any action takes place.

Trinity has blasted the move, branding the plans for action as "extraordinary" because of the low turnout at the ballot box.

Unite called for the ballot in response to Trinity implementing a nationwide pay freeze last year. According to Unite, in the past, individual sites have been allowed to stand and fall based on their own success rather than as a whole.