Ryanair in a statement angrily rejected the charge that the closures were targeted at unions, saying it had been forced to act by higher oil prices and falling airfares that have also contributed to base closures at other carriers in recent months.

The Irish airline, Europe's largest low-cost carrier, has said it hopes to secure deals with several pilot unions in the coming months in an effort to bring a wave of industrial rest under control.

But the European Cockpit Association (ECA) said the decision by the airline to close bases in the Dutch city of Eindhoven and Bremen in Germany, while also reducing capacity in the German region of Niederrhein, threatened to "seriously exacerbate" tensions.

"Closing down a base and moving your employees to a different country is not compatible with social dialogue. We see it as a declaration of war, and totally contrary to all claims to be willing to negotiate," the president of the Dutch pilots’ association VNV, Arthur van den Hudding, was quoted as saying in the ECA statement following a meeting of unions last week.

The head of Germany's Vereinigung Cockpit (VC) union, a key target for Ryanair management in its efforts to reduce industrial tensions, said his union was calling on Ryanair to change its "confrontational and counterproductive" approach.

"If Ryanair is serious about reaching agreements by Christmas, such behaviour is very unhelpful," VC President Martin Locher said in the statement. He described the Nov. 5 date for the closures as a "milestone to test management's real intentions."

Ryanair said the closures were part of a wave of cutbacks in Europe forced by higher air fares. It cited the closure of Lufthansa's Dusseldorf base, Wizz's Polish Poznan base, and Norwegian's withdrawal from Belfast and Edinburgh.

"These regrettable closures are due to adverse market conditions, and we dismiss these false claims made by Lufthansa pilots under the banner of the ECA," Ryanair's Chief Marketing Officer Kenny Jacobs said.

"The ECA and these pilot unions are silent when Lufthansa, Wizz, and Norwegian close bases or cut schedules," he said.

Ryanair announced plans to cut its Irish fleet by six aircraft earlier this year but revoked the decision after pilots backed a deal with management.

(Reporting by Conor Humphries; Editing by Mark Potter and Keith Weir)

By Conor Humphries