Lufthansa cabin crew will begin their planned strike on Friday 31 August.
German flight attendant union UFO says that for "tactical reasons" it will reveal neither the time nor the location of the walkout - but it insists that concrete industrial action will start tomorrow.
The union plans to inform its members six hours before the strike which airline stations will be affected.
UFO chairman Nicoley Baublies told German reporters that the walkout will initially get underway at individual locations, before possibly broadening to cover a wider area.
UFO says on its website that "some people might be surprised to see that the strike will take place where the responsibility for the current disaster with Lufthansa began". It is unclear whether this is a reference to Berlin Tegel airport.
Lufthansa had planned to employ temporary staff from an external job agency, AviationPower, as cabin crew on its restructured operations in the German capital.
But according to UFO, the airline could not find enough staff for the advertised positions, and its plans were disrupted by the delayed opening of Berlin Brandenburg airport - now pushed back to March 2013.
The use of external cabin crew in Lufthansa's restructured European operations, which are due to begin in January, remains a central issue in the disagreement between the airline and union.
UFO says that its industrial action could go on for "very long".