Pilots at Aer Lingus have begun an overtime ban which, according to their union, could affect flights within a fortnight.
The IMPACT Assistant General Secretary, Michael Landers, said the airline's 440 pilots were taking the action in support of around 30 colleagues who have not been rostered to fly since September.
Mr Landers said other pilots were doing overtime to maintain the airline's service.
IMPACT has accused Aer Lingus of a crude attempt to create a perception of a staff surplus.
They say that if the pilots do not maintain their flying duties, their licences and medical fitness certification will lapse.
A spokesperson for Aer Lingus said that the airline has a surplus of pilots.
The spokesperson confirmed the work-to-rule had commenced, but said that as the matter was in the middle of a process with facilitator Phil Flynn, it would be inappropriate to comment at this time.
She stressed that the schedule is currently operating normally.
Other sources say management and unions will meet Mr Flynn tomorrow in a bid to resolve the row.
Meanwhile, IMPACT is expected to confirm on Tuesday whether 1,100 cabin crew at the airline are likely to proceed with industrial action.
They have already balloted almost unanimously in favour of such action over alleged breaches of rostering agreements, and difficulties in claiming annual leave due.
If that ballot is approved by IMPACT's executive on Tuesday, it is likely that the cabin crew will serve notice of industrial action, initially in the form of a work-to-rule.
