Unions in Aer Rianta are to ballot for industrial action at Dublin, Cork and Shannon airports over the planned break-up of the company.
The decision to ballot follows a refusal by the company to provide guarantees about conditions of employment in the event of the break-up going ahead.
The ballot by SIPTU, Impact, Mandate and the TEEU, which between them represent nearly 2,000 Aer Rianta workers, is to be completed early next month.
No date for strike action has been set, however, and it is only likely to ensue if the break-up goes ahead without acceptable guarantees on the security of jobs and conditions.
The unions' move was considered a surprise, given that talks with the Department of Transport on the future of Aer Rianta are due to recommence on September 9th.
A union source, however, said the intention was simply to prepare the ground for strike action should it prove necessary.
Unions are strongly opposed to the plan by the Minister for Transport, Mr Brennan, to dismantle Aer Rianta and replace it with separate management boards for the the State's three main airports.
Workers demonstrated their anger on July 14th by staging general meetings which disrupted services for an hour, causing further knock-on delays.
Union leaders subsequently held a meeting with Mr Brennan and senior officials of his Department, and both sides agreed to a "period of reflection" and further talks.
Since that meeting, SIPTU wrote to Aer Rianta asking it to confirm that in the event of the break-up taking place, workers' conditions of employment would be guaranteed.
The company replied that as the break-up would require legislative change, and it could not be certain of the outcome of this process, it was not in a position to give the guarantees sought.
As a result of this, unions now consider that they are in a trade dispute with their members' employer, which provides legitimate grounds for industrial action. The ballot, for action up to and including strikes, is to begin next week.