SIPTU said it is "more probable than possible" that there will be industrial action at Aer Lingus over the coming weeks in a row over pensions.
SIPTU organiser Dermot O'Loughlin claimed that talks aimed at resolving the €750m deficit in the Aer Lingus/Dublin Airport Authority pension scheme ended in disarray last night.
However, this claim has been disputed by Aer Lingus.
The airline accused the union of issuing misleading statements regarding what it called "some apparent breakdown".
The union deferred threatened strike action two weeks ago to allow negotiations on the pension row to take place at the Labour Relations Commission.
Mr O'Loughlin blamed the breakdown of the LRC process on a refusal by Aer Lingus representatives to meet directly with unions representing the staff pension committees at the two companies.
He said shop stewards would be meeting in the next few days to decide when to serve notice of industrial action, and what form it would take.
However, strike notice would only be served on Aer Lingus and not on the DAA, where he said negotiations were continuing.
Mr O'Loughlin said staff were angry and disillusioned at what he described as the discourteous behaviour of the Aer Lingus management team.
Aer Lingus described claims that it had refused to meet trade union representatives, withdrawn or disrespected the negotiations process as entirely incorrect.
In a statement, it said the airline had provided clarification of some aspects of previous proposals.
It added that the Labour Relations Commission has asked it to engage with the unions' actuarial advisors on Monday and Tuesday of next week, and it is planned that that engagement will take place.
Thursday 18 October has been provisionally set as the next date for all the parties to meet at the LRC.
The airline said it remained committed to finding a resolution to the funding issues in the pension scheme.
In a statement to the stock exchange earlier this week, Aer Lingus warned that if the deficit was not addressed, members who had not yet retired might only receive 4% of the pension benefits they were expecting.