The Dutch pilots' union VNV was the latest to announce it is joining the strike against Ryanair on Wednesday.
In December, Ryanair agreed to recognise trade unions for the first time but has seen strikes in some of its biggest markets including Ireland, Spain and Italy as it struggles to reach collective labour agreements.
Last night the company tweeted confirmation that pilots based in Belguim and Sweden have announced they will strike tomorrow week, the tenth of August, while unions representing pilots in Germany and the Netherlands have indicated they may strike on that date as well. The Irish cancellations alone will affect around 3,500 passengers.
Ryanair pilots in Ireland have called a fifth one-day strike for August 10, leading the Irish LCC to cancel a further 20 flights and adding to a wave of European industrial action as a conflict with its workforce continues.
Unrest surfaced at Ryanair following a planning mix-up in September 2017 which led to 20,000 flights being canceled.
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"There is going to be disruption, it will be small, we will manage it".
Ryanair said in the statement that it was this tiny handful of Aer Lingus pilots who were behind the scenes for inciting the strikes and sabotaging the talks.
However, the dispute has escalated since Ryanair announced plans last week to move planes from Dublin to Poland, which could cost 300 jobs, including 100 pilots.
Meanwhile, Ryanair said it has agreed to nine of Forsa's 11 demands and that Forsa is repeatedly declining its invitations to meet.
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.
Ryanair refused to say anything on the lawsuit, saying it did not "comment on rumour or speculation".