Siptu, which represents 1,800 Aer Lingus workers, or around half the staff, has served notice of action to expire this Wednesday, one day before the company plans to introduce reduced terms and conditions for existing staff.

"The company will pull the trigger on this one. If it introduces the new contracts for existing staff on 1 March, then we will react, " said Siptu's national industrial secretary, Mick Halpenny.

Industry sources warned that morale in the privatised airline is at an all-time low and there are some concerns about the direction in which management is taking the former state airline. There is a feeling now that Siptu's action needs to go ahead if only "to lance the boil", said one industry source.

However, the union has so far refused to tell Aer Lingus what form any industrial action will take.

The Siptu disputes committee is to meet tomorrow afternoon to decide on the precise form of action it will take if the new contracts are introduced.

While the proposed action covers everything from an all-out strike to protest stoppages, a four-hour stoppage is considered the most likely.

Siptu says it was given assurances by Aer Lingus management before privatisation last year that any changes in the terms and conditions of employment would take place only following consultation and agreement.

But it says the company has reneged on that promise by saying it plans to introduce new and inferior contracts of employment by 1 March, with or without agreement.

Siptu claims that, while it is willing to sit down and talk about change, current contracts must be honoured and no changes can be introduced while negotiations continue.

Siptu also hotly disputes a recent report that general operatives in Aer Lingus earn 110,000 and take 20 days sick leave a year.

"The actual position, according to Aer Lingus's own documents, is that general operatives earn on average 33,000 a year or 43,000 including shift premium, rostered duty allowances, bank holiday premium and overtime, " the union said.

Impact, which represents cabin crew and management at Aer Lingus and which is facing the same changes this week, has adopted a different approach to the dispute. It is currently in talks on management's agenda, though it is understood there is unlikely to be any breakthrough before next Wednesday. But Impact has not served notice of action.

Aer Lingus pilots, who are also members of Impact, are involved in a separate negotiating process under the chair of industrial relations troubleshooter Phil Flynn.