Ryanair confirmed to AFP that 150 of the 400 flights normally departing from Germany on Wednesday had been cancelled.
Those cancellations have had a knock-on effect in Denmark, with both Copenhagen and Billund airports announcing cancellations on their websites.
The action has not been declared a strike, but is rather described as a protest ‘walk-out'. The pilot association in Germany and trade union Verdi have organised the action, Ritzau writes.
Ryanair has described the protest act as “unnecessary” and “unacceptable”, AFP reports.
Similar actions from Ryanair employees in Belgium, Italy, the Netherlands, Portugal and Spain in the near future have also been mooted.
Last autumn and winter, the low-cost airline was forced to cancel as many as 20,000 flights, affecting up to 700,000 passengers, due to industrial action by employees.
A lack of pilots in Europe resulted in Ryanair's flight staff asking for better terms. Ryanair dismissed pilot demands, which led to the subsequent staff action.
The company has since begun to reach agreements with trade unions in a number of countries, despite strained relations with employee organisations.
A recent agreement with Ireland was reached after Ryanair said it was considering moving a base and 300 jobs to Poland.
“That is how Ryanair negotiates with its staff – by scaring them and threatening them with job losses,” Markus Wahl, the vice-chair of Germany's pilots' union, said.
The airline said on Tuesday that further action could result in flights being moved and jobs being scrapped.
“We are not making threats. If there are ongoing flights, there will be an economic effect,” the company's marketing director Kenny Jacobs said according to AFP's report.
Details of cancelled flights in Denmark can be checked on the following airport websites:
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