Hundreds of Ryanair flights - including at least 50 services into and out of Britain - will not take off as planned today due to pilot strikes in five countries.
The Irish budget airline said around 55,000 passengers would be affected by the strikes, and they have been offered refunds or the option of rerouting their journey.
Officials said flights should return to normal Saturday.
The Dutch pilot union, VNV, had earlier this week described today's strike as a "wake-up call" for Ryanair.
Last month cabin crew on Ryanair flights serving Belgium, Spain, Portugal and Italy downed tools.
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A judge later ruled that Dutch pilots could join the strike.
The Irish airline, Europe's largest low-priced carrier, averted widespread strikes before Christmas by agreeing to recognise unions for the first time in its 30-year history.
But Ryanair pilots say they earn less than counterparts at other airlines such as Lufthansa.
French news source France Info dubbed the strike "historic", specifying that it is the biggest walkout in the history of Ryanair.
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Another key complaint of workers based in countries other than Ireland is the fact that Ryanair employs them under Irish legislation, arguing most of its employees work on Irish planes.
German pilot representatives said on Wednesday they were joining the strike action because they want pay and work conditions comparable to those at Ryanair's competitors. "It is annoying that it's happening in the holidays but that is the only means they have", said one man at an airport in Berlin.
That was because the strikes were hurting bookings, Ryanair said, and although it was too early to assess the impact elsewhere, it added that the action will hit average fares from having to move customers to flights it could otherwise have sold at a high last-minute price.
At Charleroi Airport, Belgium's second largest and a major Ryanair hub in the region, striking staff gathered in the departure hall and held up banners reading "Ryanair must change- Respect us".
"We want to again apologise to customers affected by this unnecessary disruption and we ask the striking unions to continue negotiations instead of calling anymore unjustified strikes".
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