Ryanair has warned that it may slash jobs and close some bases in Germany if it is hit with more strikes, a day before a planned work stoppage for better pay and conditions.
The airline said it has cancelled 150 of 400 flights to and from Gemany tomorrow over the "surprise and unnecessary strike".
The company said it has notified affected customers and "offered a refund, free move on to the next available flight or reroute options".
"We apologise sincerely to our customers for this unnecessary strike and regrettable disruptions which Ryanair has done everything in its power to avoid," said Ryanair chief marketing officer Kenny Jacobs.
"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," said Mr Jacobs in a statement earlier today.
Without naming the German sites under threat, Mr Jacobs said they are already loss-making during the winter season and could suffer greater losses if strikes continued.
Condemning the cockpit pilots union's call for a 24-hour walkout that is expected to involve 400 pilots and co-pilots in Germany, Mr Jacobs also rejected the charge that the airline is underpaying its staff.
Lufthansa's subsidiary Eurowings pay pilots 30% less, he claimed.
"It is unacceptable that a union representing Ryanair's German pilots, who earn up to €190,000 a year and work a five-day week followed by a four-day weekend, is now threatening customers travel plans at short notice and without consulting with our German pilots," said Mr Jacobs.
Tomorrow's stoppage comes after a round of coordinated industrial action by pilots in Ireland across Europe in August that forced the airline to cancel 400 out of 2,400 scheduled European flights during the peak summer holiday season.
Germany was worst hit by the August strike, with 250 flights scrapped at ten airports.
Since then, Ryanair, which carries 130 million passengers annually, has reached agreements with personnel in Ireland and Italy.
The airline backed down after the agreement from an announcement that it would move some aircraft out of Ireland and slash 300 jobs.
However, Germany's Cockpit union has rejected the deal.
Ryanair 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.
