Aer Lingus pilots are balloting for industrial action, including a withdrawal of service in a dispute over rosters.
The ballot by the Irish Airline Pilots' Association will not be completed until 27 May.
The ballot paper issued to pilots has said that they will engage in industrial action if Aer Lingus management fails to address pilot grievances over rosters on both the A320 and A330 fleets by 7 June.
IALPA is seeking that the airline produce and implement a sustainable manpower plan to address issues, including pilot number retention.
Pilot sources have said that there is now a serious shortage of pilots, as too many had been allowed to leave the airline.
They said pilots were being forced to work rosters that were execessively harsh and that the airline was hiring in contract pilots due to the shortfall in staff numbers.
No one from IALPA was available for comment.
Aer Lingus have said that they had been notified that the pilots were balloting for industrial action.
A spokesperson said they were in the ongoing process of recruiting pilots for training.
Aer Lingus has said it is taking on a number of first officers or co-pilots on short-term contracts.
The spokesman said that this was not a new development and was permitted under existing agreements.
He also said that the two sides were continuing to engage in a bid to resolve the row and had met today.
