Siptu president Jack O'Connor has accused Aer Lingus management of "blackmail" following the company's announcement that it was imposing a pay freeze.
Aer Lingus has said it will not pay money owed to workers under the national partnership agreement Towards 2016 until a cost saving plan is implemented.
Mr O'Connor said the pay freeze amounted to a 12 per cent pay cut in real terms and said that the bonus scheme management voted through in July was being paid for by the workers.
"Now we see how this bonus scheme is going to be funded by not paying ordinary workers, many of whom are on average wages or less, the terms of the pay increase . . . which is just about enough to compensate of the consequences of inflation," said Mr O'Connor.
"We recognise that the practices in the aviation industry are increasingly being determined by some of the worst employers in the country and that Aer Lingus has to compete with those; nonetheless, it is not an excuse to abandon all civilised behaviour and practices," he added.
The Impact union said the pay freeze was "completely unacceptable" and that it had been "fully engaged" in the Labour Court process to reach agreement on the airline's cost saving plans.
"The company's announcement of a pay freeze represents a heavy handed and deliberately provocative approach, demonstrating an outrageous disregard for the tried and tested industrial relations machinery of the State," the union said.
Various branch committees of Impact will be meeting in the coming days to formulate a response to the company, Impact added.