Spanish pilots union Sepla has announced it is suing the Irish airline in Spain's High Court over its pilot contracts. The strikes will coincide with stoppages by colleagues in Sweden and Belgium who announced their plans to strike on Wednesday.
Ryanair has said it may face industrial action in Germany and the Netherlands on the same day.
Ryanair's chief people officer, Eddie Wilson, said: "Given the non-engagement by Forsa, and the manipulation of the discussions by certain Aer Lingus pilots to ensure that no meetings take place, that unsuccessful strikes keep repeating, Ryanair now feels the only way to introduce common sense is by way of third party mediation, and is suggesting Mr Kieran Mulvey, formerly of the Labour Commission and Workplace Relations Commission".
Cabin crew in Italy, Spain, Portugal and Belgium staged a two-day strike that forced Ryanair to cancel the flights of more than 50,000 customers, the airline's biggest-ever strike.
Forsa should explain why, "when Ryanair has agreed nine of their 11 requirements, they call strikes rather than negotiate", the statement said.
Kenny Jacobs, chief marketing officer at Ryanair, commented: "Regrettably, nearly 200,000 Ryanair customers had their flights cancelled in July because of repeated ATC staff shortages in the UK, Germany and France, adverse weather, and unnecessary pilot and cabin crew strikes".
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Twenty-five per cent of Ryanair's crew are striking today, marking its fourth strike so far.
The airline is bracing itself for another round of strike action next Friday, when 25 per cent of its pilots will strike.
"We now call on Forsa to accept Mr Mulvey as a third party mediator, an initiative Forsa have repeatedly called for in recent weeks", he said.
The first strike was followed by two more one-day strikes on July 20 and July 24.
Ryanair said it has notified all customers affected and is providing refunds or alternative flights.