The increasingly bitter dispute between Aer Lingus staff and management worsened tonight after four pilots were suspended for refusing to train new recruits.
The officers were told not to come back to work until they had agreed to mentor air crew for the controversial Belfast base.
A captain, who works out of Dublin, was suspended by the airline earlier today, while three further suspensions were announced tonight.
Around 30 others, who had an optional agreement to train new recruits, have been told by their union Impact to resign from that role but continue day-to-day flying duties.
An Aer Lingus spokeswoman said: "These pilots have been appointed as instructors. "This is a paper exercise to evade their responsibilities. "We will not accept partial resignations of responsibilities," she said.
In a statement Impact, parent union of the Irish Airline Pilots Association (Ialpa), said putting pilots off planes was self-defeating and inevitably risked disrupting services. Impact said customers would likely book with other airlines.
"Pilots who have training-instructor responsibilities have started to resign these functions," a union spokesman said.
"This does not require them to resign as pilots because training-instructor responsibilities, which attract an additional payment, are optional and pilots are entitled to take them on or give them up."
Impact said it was open to talks either directly with Aer Lingus management or through the state's industrial relations machinery. Ialpa's executive committee is holding meetings with pilots threatened by suspension.
But Aer Lingus said it had not received a request from the pilots' union today to meet.
"Should they \[Aer Lingus] receive such a request, they will only agree to meet if the policy of non-cooperation with Belfast is lifted first," a company spokeswoman said.
Earlier, Taoiseach Bertie Ahern pleaded with both sides to agree a resolution in the long-running dispute over the Belfast-Heathrow route and avoid an escalation in tensions.
The intervention was welcomed by Impact. Mr Ahern said management and unions should work to avoid confrontation.
"Where there are problems the best way is to work together to resolve them," said Mr Ahern. "And I would just say, without in any way wanting to hype this issue, both sides should understand it is in their own collective best interests. "There's never winners and losers in disputes and difficulties.
"If there are difficulties and problems the only thing that has to ultimately happen is resolutions of them. "And I wish both sides well in trying to do that."
Pilots were warned in a letter from Aer Lingus chief executive Dermot Mannion to help with the selection and training of new staff by today or face suspension without pay.
It comes just days after the carrier announced a pay freeze for employees as part of its €20 millioncost cutting plan, over which fellow union Siptu threatened strike action.
In the letter Mr Mannion said the airline's pilots were needed to help train applicants to the captain and co-pilot positions at the new Belfast hub.
"Assisting with recruitment and/or training of new staff is an integral part of the duties of a significant number of pilots and full co-operation is expected in this regard," he wrote.
"It is important that you understand that if you fail to carry out your full range of duties, for whatever reason, you will be suspended from the payroll with immediate effect."
Mr Mannion, who rejected claims the company was adopting a confrontational stance, said pilots' concerns over terms and conditions for staff at the new hub had been resolved by the airline. Impact trade union welcomed the Taoiseach's comments and called for further talks with management.