Lufthansa is facing major disruptions to its flight operations on Sept. 4, after the UFO cabin crew union announced another strike.

UFO initially did not say where and when its members would stage their protests, but later revealed plans for strikes at both Frankfurt and Berlin. UFO’s head Nicoley Baublies says the protest action will be more severe than on Aug. 31, when Lufthansa had to cancel around 200 flights because of an eight-hour walkout at its main base in Frankfurt.

The airline and its union are in a bitter dispute that is mainly about outsourcing. Lufthansa wants to set up a separate subsidiary to take over part of its loss-making European operation. The carrier wants lower labor costs across the group or a lower pay scale for new hires that are to be dedicated to the subsidiary. UFO is opposed to any outsourcing. Lufthansa is offering a 5% pay increase in return for significant productivity concessions.

UFO says that it will give the company some time to react following the strike. If Lufthansa does not come up with a new proposal, the union plans to call for an unlimited strike across all stations.