Lufthansa scrapped 930 more flights on Wednesday, grounding 100,000 passengers after a court allowed cabin staff to press on with a strike that is shaping up to be the worst in the German airline's history.
The new flight cancellations in and out of Germany's biggest airport, Frankfurt, as well as hubs in Munich and Duesseldorf, come after four days of stoppages that had already forced the airline to cancel 2,800 flights, leaving 300,000 passengers stranded.
The industrial action over a dispute with the UFO flight attendants union regarding cost cuts began on Friday with a pause on Sunday, and is set to go on until Friday.
Lufthansa had sought to challenge the legality of the industrial action through court injunctions but lost a key bid after the labour court of Darmstadt, which has jurisdiction for Frankfurt and Munich airports, approved the strike.
The court dismissed the airline's argument that the union's reasons for striking were "too vague" and unjustified under German labour law.
The UFO union welcomed the ruling on its website, calling for the strike to continue from 0300 GMT on Wednesday until 2300 GMT on Friday across all flights at Frankfurt, Munich and Duesseldorf airports.
Lufthansa in a statement said it stood by its position that "the reasons for the strike were not defined clearly enough" and said it will decide its "next steps" on Wednesday.
The setback came after an earlier symbolic victory for the airline dealt by another court which ruled that the strike affecting Duesseldorf airport was illegal - but that judgement was effectively useless as it was limited to Tuesday only and came hours after flights had been cancelled.
The same court is due to issue later Wednesday a ruling for the remaining period of the strike.
The airline has been locked in a battle that erupted nearly two years ago with cabin crew over early retirement provisions which the union wants unchanged.
But Lufthansa argues that the system is too expensive in the face of cut-throat competition from low-cost operators such as Ryanair and Easyjet.
Late Monday, the airline presented a new offer to the union, with improved bonuses and retirement provisions, but UFO called it unacceptable.
This is the first time that cabin staff have staged walkouts in the long-running dispute, which does not affect Lufthansa's subsidiaries Germanwings, Eurowings, Lufthansa CityLine, SWISS, Austrian Airlines, Air Dolomiti and Brussels Airlines.
It is also separate from another tussle between management and pilots over the company's plans to change the early retirement arrangements for pilots.
Lufthansa wants to scrap an arrangement under which pilots can retire at 55 and receive up to 60 per cent of their pay until they reach the statutory retirement age of 65.
The pilots, who are concerned about Lufthansa's aim to further develop its low-cost activities, have staged repeated walkouts since 2014, costing the airline 350 million euros (US$375 million).
Despite the competitive airline environment, Lufthansa said last week it was raising its full-year forecasts after low oil prices and positive passenger numbers lifted profits in the third quarter.