They are members of Unite Trade Union and the Connect union.
Unite had voted to accept a Labour Court Recommendation it had previously rejected.
However, it now says management have not agreed to implement the deal.
Willie Quigley, from Unite, says they will now hold a vote on whether to take strike action:
"I advised the employer on Monday of this week that Unite members had accepted the recommendation and we wished for the implementation of that to proceed and the options that were with it to be made available. The company wrote to me on Tuesday, telling me that they were now effectively disowned the court recommendation, and were no longer going to implement it, and they had withdrawn everything off the table."