Now, 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.
Ryanair's call for talks with its Ireland-based pilots' union Forsa over labour disputes has been rejected, said the airline on Thursday.
Pilots in Belgium and Sweden have announced a walkout for Friday, August 10.
The Irish pilots, who took three days of industrial action last month, are demanding better working conditions.
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"For over a month, the union has said that industrial action is likely to continue until there is substantial movement on the pilots' reasonable demands for an agreement on a fair and transparent approach to base transfers and related matters", Fórsa said.
Ryanair previously invited Fórsa to talks this coming Tuesday, August 7, in a bid to prevent strike action at Dublin Airport's Terminal 1.
Pilots in Germany and the Netherlands are considering joining in.
Ryanair, which operates from 86 bases in 37 countries and carried 130 million passengers last year, averted widespread strikes before last with a last-minute U-turn to recognise unions for the first time in its 32-year history.
Spanish pilots union Sepla has stated that it is suing strike-hit Ryanair in Spain's High Court over its pilot contracts.
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20 flights in total have been cancelled, meaning 3,5oo passengers will have to rearrange their travel plans.
However the trade union FORSA says the airline's threat last week to sack 300 pilots and cabin crew or transfer them to Poland has led to a hardening of resolve among its members.
The first strike was followed by two more one-day strikes on July 20 and July 24.
Ryanair cancelled 16 of around 2,300 daily flights on after the third one-day strike by Irish pilots.
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