Dutch and German pilots with Ryanair are set to strike on Friday - bringing the total number of countries affected by work stoppages to five.
Pilots based in Belgium, Sweden and Ireland have previously announced their intention to stop work for 24 hours on August 10th.
The strikes have already seen the airline cancel around 400 flights to/from the five countries.
The Dutch Airline Pilots Association (VNV / Dutch ALPA) is the latest union to confirm their plans.
In a statement, the association said: "For the last eight months the Dutch Airline Pilots Association has tried to negotiate a collective labour agreement between Ryanair and the Dutch ALPA without any result.
"This European pilot strike should be a wake up call for the Ryanair management."
It adds: "The Dutch ALPA demands are modest: Dutch law should be applied; no more bogus self-employment and a sufficient sick pay and pension."
It was earlier confirmed the German strike will run for 24 hours from 3.00am on Friday.
In response, the company said it was forced to cancel 250 out of over 2,400 flights scheduled for Germany on Friday.
It said the German Vereinigung Cockpit pilots union had failed to give seven days notice ahead of the strike, leaving the airline unable to minimise customer disruption.
All affected passengers will be informed - and will offered a refund, reroute or free move.
Members of the German Vereinigung Cockpit pilots union announce plans to strike on Friday, 08-08-2018 | Image: Frank Rumpenhorst/DPA/PA Images
'Only Ryanair is responsible'
Announcing the decision to strike, Vereinigung Cockpit president Martin Locher said: "We demand improvements in pay and working conditions.
"Improvements are inconceivable without an increase in personnel cockpit costs.
"During negotiations Ryanair categorically ruled out any such increases.
"At the same time, Ryanair has not shown any interest to find solutions. It is only Ryanair, which is responsible for the escalation which has now taken place."
In a statement, Ryanair called on the union to "meet and continue meaningful negotiations rather than call strikes."
File photo of Ryanair Chief Marketing Officer Kenny Jacobs, 06-09-2018 | Image: Rainer Jensen/DPA/PA Images
Spokesperson Kenny Jacobs said the strike action is "unnecessary" and insisted the airline had sent the union revised proposals last Friday.
He said the airline had requested a meeting with the union yesterday, "but they did not respond to this invitation."
"Our pilots in Germany enjoy excellent working conditions," he said.
"They are paid up to €190,000 p.a. and, as well as additional benefits, they received a 20% pay increase at the start of this year.
"Ryanair pilots earn at least 30% more than Eurowings and 20% more than Norwegian pilots."
The German and Dutch strikes coincides with pilot strikes in Ireland, Belgium, and Sweden.
Some 20 out of 300 Irish flights have been cancelled, affecting 3,500 passengers.
104 flights to/from Belgium have been cancelled, along with 22 flights to/from Sweden.
Last Friday some of Ryanair's Irish pilots held fourth strike.
The company had proposed third party mediation in a bid to resolve its dispute with Irish pilots.
It comes amid a continuing dispute with the trade union Fórsa, over issues such as seniority and base transfers.
In a statement, Ryanair proposed former head of the Workplace Relations Commission (WRC) Kieran Mulvey as a third party mediator.
It is understood Mr Mulvey is not available until next week.
With reporting from Michael Staines and Stephen McNeice