Ryanair pilots could take industrial action, as the carrier cancels thousands of flights.
The carrier is trying to clear a backlog of annual leave for pilots, having offered them bonus payments of up to €12,000 to forgo their holidays.
But they've have rejected the proposals.
Workers have also discussed industrial action, with a draft letter being sent to their European bases.
The Irish Examiner reports the letter, signed by Ryanair pilots, has warned they will 'work to rule'.
"We would like to advise that with immediate effect, the pilot workforce at the bases [airports] listed below rescind the goodwill that has been extended toward the company for many years, including working days off and turning up early. In short, we shall now ‘work to rule," the letter says.
Ryanair's chief executive Michael O’Leary will host a media briefing at Ryanair HQ in Swords later.