Aer Lingus management was tonight accused of attempting to blackmail its staff into accepting tough cost-cutting measures.
The airline, now facing its second major dispute in as many months after abandoning the Shannon-Heathrow service, has frozen three pay rises worth 7.5% as it strives to save around €20m .
Both Siptu and Impact condemned the plan, branding it completely unacceptable and provocative.
"Aer Lingus has sunk to a new low in threatening our members with a pay freeze because they haven't agreed to the company's cost-cutting plan," said Micheal Halpenny, Siptu's national industrial secretary.
"Our members are not going to succumb to this kind of blackmail."
Impact's Niall Shanahan said the move was completely unacceptable.
"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," he said.
Staff were informed of the controversial move in a letter from Aer Lingus chief executive Dermot Mannion.
He said management had run out of patience with unions following a 10-month battle to encourage savings and implement a radical cost-cutting plan.
The airline's Programme for Continuous Improvement (PCI) is designed to save €20m euro by modernising working arrangements.
The freeze covers three increases, a 2.5% rise due yesterday and again next April, both under the Towards 2016 social partnership agreement, and a further 2.5% annual payment.
Mr Mannion said existing staff costs leave the airline at a serious competitive disadvantage.
"Aer Lingus cannot continue to wait indefinitely to achieve essential savings. It is within this context that the decision to suspend increases has been taken," said Mr Mannion.
"I suppose what we would say is we are doing this because we have reached the end of the line.
"We have run out of patience, there is no further time for discussion. We must now move to fully implement the terms of the PCI."
The airline backed up its case for cost-cutting with claims that some departments have more supervisors than staff needing to be supervised, ground staff can earn €110,000 a year, and some staff finish shifts early but get paid the full rate, while part-timers get full pension entitlements.
Siptu said the cuts could wipe €5,000 from some wage packets while Mr Mannion would earn €982,000 this year.
Mr Shanahan said cabin crew were awaiting the outcome of the latest Labour Court hearing on the issue which was held in August.
"Any assertion that these avenues have already been exhausted, given that a decision by the Labour Court is still pending, is disingenuous and misleading, and completely disregards the union's efforts to reach a solution," he said.
It is understood unions were warned yesterday the airline was prepared to call a pay freeze.
Impact representatives are to meet within the next 48 hours to decide how to respond to the move. Siptu representatives will also meet and are expected to call for face-to-face talks with Mr Mannion.
The unions have the option of asking the Government's chief industrial relations troubleshooters at the National Implementation Body to assess whether the pay freezes breach the Towards 2016 social partnership agreement.
Impact represents 1,320 cabin crew and 534 pilots at the airline.
In the letter to employees, Mr Mannion said the PCI was an essential cost-saving initiative, addressing terms and conditions of employment out of line with industry norms and inefficient working practices which adversely affect productivity.
He said it was a vital element of the airline's plan to allow Aer Lingus to remain an independent, stand-alone business and fulfil future expansion.
Its aim is to bring down the cost per passenger. It focuses on a range of costs including staff, fuel, maintenance and airport handling.