Aer Lingus employees are to be balloted on industrial action after a letter from management signalled they must accept new work practices.
In the letter, believed to have been posted on noticeboards at Aer Lingus last night, the company's chief executive Dermot Mannion warns that the airline is facing "commercial challenges and threats that are unprecedented in the history of the company".
The letter warns that Aer Lingus is out of line with its competitors in "some critical areas". It goes on to state: "Given our significant growth and expansion plans this is no longer sustainable."
Aer Lingus does not specifically state what will happen if the plan is rejected
but warns that it will be forced "to address the issues by all available alternative means". Unions believe this means the airline will cut jobs if the new work practices are not accepted.
Mr Mannion said the changes must be introduced by February 1st for new employees and by March 1st for existing employees, according to media reports.
Staff on new contracts would get less pay for overtime and would be entitled to fewer holidays.
The main unions at Aer Lingus, Siptu and Impact have warned they will not accept the airline's plan to unilaterally change work practices. The dispute is expected to be referred to the Labour Relations Commission.
Aer Lingus has seen some 3,000 jobs cut in recent years, bringing staff numbers down to around 3,500 from 6,500 just five years ago.