A number of smaller unions in Aer Lingus are to follow Siptu's lead in balloting for industrial action over the plan to privatise the airline.
Approximately 2,000 Siptu members in the company began balloting yesterday on a strike, to be held in the event of changes being imposed on workers.
A result of the ballot is expected next week but there are no immediate plans to implement industrial action.
The remaining five craft unions in Aer Lingus, representing more than 200 workers, decided yesterday to ballot members on the same issue.
Brian Gormley of Amicus, one of the unions involved, said they were not seeking an industrial dispute, but did want the Government and the airline's management to listen to their views.
"We are concerned about job security, pay and conditions," he said. He added that concerns about the airline's pension fund had been used as a "red herring" to justify the sale of the airline.
"The pension fund is not in deficit. The problem it has is due to inadequate ongoing revenue because job cuts have reduced the size of the workforce," Mr Gormley said.
"The contributions issue will not be solved by a once-off cash injection. Our members are not looking to become wealthy shareholders, they are simply looking to keep their jobs."
Members of Siptu are balloting for industrial action "in the event of Aer Lingus management unilaterally moving on core issues of concern to members such as their terms and conditions of employment, job security and pensions".
Siptu national industrial secretary Michael Halpenny said the Government's own advisers had said a sale of the airline would realise only €400 million.
This meant the national airline would be delivered to private investors "for the price of four planes", while Aer Lingus would still have to "chase the balance" of the €2 billion it required for investment purposes, he said.