Siptu members have voted overwhelmingly to reject a set of proposals aimed at resolving the dispute at Aer Lingus over cost savings.
The airline is seeking to introduce new, more flexible work practices for about 1,800 workers which it believes will save about €10 million a year.
But a deal over the new arrangements, agreed by management and Siptu representatives last month, was rejected by three to one, the trade union said in a statement tonight.
National Industrial Secretary Gerry McCormack said: "The union has put a huge effort and resources into trying to secure agreement in the different local areas on cost cutting measures.
"However, the company's failure to produce workable rosters in line with the proposals has contributed significantly to the decision by our members to reject them," he said.
Under the plan, staff would have had to work shifts of varying lengths while the company will have the freedom to move personnel between duties, for example between check-in and boarding, in the course of a shift.
It is understood that about €7.5 million will be generated in savings at Dublin airport and €2.5 million in Shannon.