Ryanair warned Tuesday that it may slash jobs and close some bases in Germany if it is hit with more strikes, a day before a work stoppage by pilots and cabin crew for better pay and conditions.
The Irish no-frills airline has blasted the planned walkout as "unnecessary" and said it would be forced to cancel 150 out of 400 scheduled flights to and from Germany.
"These threatened strikes can only damage Ryanair's business in Germany, and if they continue, will lead to base cuts and job cuts for both German pilots and cabin crew, particularly at some secondary German bases," the airline's chief marketing officer Kenny Jacobs said.
The spat is the latest escalation in Ryanair's battles with employees across Europe since it first started recognising unions last year.
Germany's Cockpit union and the Verdi services sector union have called for a 24-hour walkout from 3:00 am (0100 GMT) on Wednesday.
The unions, representing some 400 Ryanair pilots and 1,000 crew, argue that no headway has been made with the airline's management despite repeated negotiations.
But Ryanair hit back, saying it had already offered more local contracts and pay rises for pilots in Germany.
"Ryanair is totally serious about working with unions," Jacobs told a press conference in Frankfurt.
"It is unacceptable that a union representing Ryanair's German pilots, who earn up to 190,000 euros ($220,000) a year and work a five-day week followed by a four-day weekend, is now threatening customers travel plans at short notice," he added.
Ryanair said it would try to minimise the disruption for customers, who will be offered free re-bookings.
The airline last year averted widespread Christmas strikes by agreeing to recognise trade unions for the first time in its 33-year history, but it has since struggled to reach deals with the workers' representatives.
Ryanair was hit by its first ever simultaneous pilot strike in five European countries last month, including in Germany.
The walkout forced the airline to cancel 400 out of 2,400 scheduled flights during the peak summer holiday season.
More trouble is brewing with unions in Belgium, the Netherlands, Italy and Spain calling for another mass walkout in the last week of September.
The unions have said it would be "the biggest strike action the company has ever seen". The famously low-budget airline boasts lower costs per passenger than its competitors and is eyeing profits of around 1.25 billion euros this year.