A pilots strike at budget airline Eurowings forced the German carrier to cancel hundreds of flights on Thursday.
The Lufthansa subsidiary said about half of its 500 daily flights would be cancelled, affecting tens of thousands of passengers.
German airports are heavily affected, but others such as Stockholm, Prague and Majorca have also been hit.
Pilots’ union Vereinigung Cockpit announced the walkout after talks with management about improving working conditions stalled.
Pilots are asking for the maximum number of flying hours to be reduced.
Strikes at parent company Lufthansa were called off last month after the airline and union reached a pay deal to address the effects of inflation.