Ryanair is facing its biggest strike yet on Friday, when pilots in Germany, Belgium, the Netherlands, Ireland and Sweden plan to hold a coordinated 24-hour stoppage to demand better working conditions.
Compensation can be claimed for some cancelled flights but it varies and depends on how much information was given in advance.
This is the latest of several strikes by Ryanair pilots and cabin crew in under a month, and the first involving joint action by pilots from several countries.
Ingolf Schumacher, pay negotiator at Germany's Vereinigung Cockpit (VC) union, said pilots had to be prepared for "a very long battle" and that it could take months to push through change at Europe's largest low-priced carrier.
About 50,000 passengers are understood to have been told of cancellations on these flights - as reported by AP. A year ago it agreed to recognise unions for the first time but it is in a dispute over collective labour agreements. But the figure could rise to 82 flights if routes between the Dutch city of Eindhoven and the Spanish cities of Reus and Valencia are finally canceled, after a court said on Thursday that Dutch pilots may join the walkout.
Despite the walkouts, 85 per cent of its scheduled flights, more than 2,000, will operate as normal, Ryanair said.
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'The majority of customers have already been accommodated on another Ryanair flight.
Staff claim this creates huge insecurity for them, blocking their access to state benefits in their country.
A statement from CEO Michael O'Leary said: "Ryanair fully complies with all EU261 legislation, however as these flight cancellations were caused by extraordinary circumstances, no compensation is due".
The Spanish pilots' union - which represents around 500 of the 800 Ryanair pilots in Spain - says it is going to sue the airline after a year of failed talks.
Peter Scherrer, deputy secretary general of the European Trade Union Confederation, said he welcomed today's cross-border show of unity by pilots.
"In its update, Ryanair said: "[We] took every step to minimise the disruption and we notified our customers as early as possible advising them of their free move, refund, or reroute options.
It also called on the striking unions to return to negotiations rather than "calling any more unjustified strikes".