Ryanair pilots are striking in five European countries, forcing the cancellation of a sixth of the firm's flights during the holiday season peak.
The 24-hour walk-out involves staff in Germany, Sweden, Ireland, Belgium and the Netherlands. About 50,000 passengers are understood to have been told of cancellations on 400 flights.
The action is the largest in a series of strikes over pay and conditions.
Ryanair says it has made every effort to resolve the dispute.
The Irish airline, Europe's largest low-cost carrier, averted widespread strikes before Christmas by agreeing to recognise unions for the first time in its 30-year history.
But there have been protests ever since over the negotiating of collective labour agreements.
About 300 Ryanair flights were cancelled in July when cabin crews in Belgium, Portugal and Spain went on strike for 48 hours.
The airline in recent days announced the cancellations of 250 flights in and out Germany, 104 to and from Belgium and another 42 affecting Sweden and Ireland.
A Dutch court on Thursday evening rejected a case from Ryanair seeking to block pilots in the Netherlands from joining the strike, affecting about 22 flights. But at about the same time Ryanair issued a statement saying there would be no cancellations.
The unions want the contracts of Ryanair employees to be governed by the laws of the nation where they are based, not by Irish legislation.
Ryanair has described the strike action as "regrettable and unjustified", claiming its pilots are paid more than other budget airlines, reports BBC.
It said it has taken every step to minimise the disruption, adding: "The majority of customers affected have already been reaccommodated on another Ryanair flight."
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