Striking staff have forced Ryanair to cancel around 250 flights on Friday, potentially affecting tens of thousands of passengers across Europe.
A dispute over pay and conditions between the budget airline and staff in six European countries will see them walk out for 24 hours.
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Ryanair had cancelled 150 flights due to an "unnecessary" strike in Spain, Belgium, the Netherlands, Portugal and Italy.
On Thursday the airline said pilots in Germany would also be going on strike, with up to 100 flights shelved as a result.
Ryanair's chief operations officer Peter Bellew said the action was "deeply regrettable".
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"It is deeply disappointing that some of our customers and our people in Germany tomorrow will have their flights disrupted by an unnecessary strike called at short notice," he said.
The strikes are the latest in a series of walkouts by staff at Europe's largest low-cost airline.
On September 12 dozens of flights were cancelled in Germany after pilots and flight attendants refused to work for 24 hours.
In August several days of strikes involving staff in Ireland, Germany, Sweden, Belgium and the Netherlands saw hundreds of flights cancelled.
Earlier this month Ryanair chief executive Michael O'Leary said strikes by pilots were necessary to keep air fares down.
He insisted industrial disputes with the airline's staff have caused "minimal disruption" as he pledged not to "roll over".