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Aer Lingus has begun suspending pilots refusing to train new recruits in a dispute over pay and conditions for the airline's planned new hub at Belfast.
Members of the Aer Lingus pilots' union Impact have said they will resign en masse from training duties.
It follows the suspension of four of their colleagues for refusing to train new recruits who will be employed on lower pay and pensions.
Almost 40 senior pilots have begun resigning from training duties.
By doing so they are seeking to avoid the risk of suspension for refusal to train new recruits.
The Irish Airline Pilots' Association (Ialpa) which is part of Impact wants to negotiate over terms and conditions to apply in Belfast.
However, the airline has said negotiations are over and it wants to set up its new hub north of the border employing staff on conditions "with reference to local market conditions".
Four Ialpa members have been suspended so far for refusing to take part in the training of new recruits, in line with union policy to black co-operation with management's Belfast preparations until agreement is reached.
While there was speculation that a suspension might trigger a strike - for which the union already has virtual unanimous approval from its membership - the pilots' move to formally withdraw from training is a surprise one, seeking to outfox management.
An Impact spokesman told the BBC that training was an optional extra duty beyond pilots' core flying duties, which they elected to make themselves available for and for which they received separate pay.
Resignation threat
However, the union claims that an in-house training capability was also a regulatory requirement of Irish aviation authorities, and that loss of this through mass resignations would hit the airline hard as it gears up for its new Belfast operation, while immunising pilots from disciplinary action.
The airline, which refused to comment on Tuesday, however, is expected to be examining if the pilots' withdrawal from training duties could spark any further disciplinary sanction by management.
The union has said it wants to solve the dispute without industrial action, and to negotiate, a path also urged by Taoiseach Bertie Ahern.
"Our line remains the same - we want to discuss it, we want to negotiate, we want to resolve the problem," an Impact spokesman said.
Belfast International Airport
Aer Lingus is setting up a hub at Belfast International
Privately, union members are likely to be very reluctant to strike, conscious that staff shareholdings acquired in last year's privatisation of Ireland's former national airline will be damaged if its overall financial health is affected by a dispute which could weaken the share price.
This may be why Aer Lingus management appears to be playing tough, confronting the pilots last week and also telling other staff it will not honour a pay award supposed to be paid as part of an Irish government-brokered national pay deal, unless staff accept the airline's cost reduction proposals.
A showdown may be part of Chief Executive Dermot Mannion's plans to streamline the airline, and develop a new post-privatisation culture in the face of stiff competition from bitter rival and substantial shareholder Ryanair.
The airline also faces stiff competition from rivals including Ryanair on almost all its UK and European routes, and British Midland on its routes across the Irish Sea.
With the new Belfast hub due to open in early December, the latest surprise move by pilots places more pressure on management to resolve the increasingly bitter dispute, and meet its deadline for take-off from Belfast.
Any uncertainty or postponement of the start of services from Belfast would represent a public relations disaster for the airline, already taking bookings for its advertised services to London Heathrow and European destinations from Aldergrove.
The stakes for the pilots are high too however, fearful that staff on lower pay and pensions at a base only 110 miles just up the road from Dublin could eventually reduce their own conditions, which are considerably better than those enjoyed by staff in rival Ryanair.