Insolvent airline Air Berlin, Germany's second-largest carrier, has cancelled around 100 flights after a large number of its pilots called in sick.
It prompted the airline to accuse them of sabotaging rescue talks with potential investors.
The airline filed for insolvency in August after its main shareholder, Gulf carrier Etihad, withdrew financial support.
Domestic and international flights have been affected, stranding passengers.
'Playing with fire'
Lufthansa's low cost subsidiary, Eurowings, which is renting aircraft and pilots from Air Berlin, also had to cancel flights.
Chief executive Thomas Winkelmann said the pilots in question were "playing with fire".
Some 200 of its 1,500 pilots suddenly called in sick.
Reports in the German media have said that the airline is in a dispute with its pilots about the transfer of staff to a new owner.
The action comes ahead of a Friday deadline for interested bidders to submit offers to take over parts of the carrier.
Mr Winkelmann said: "This day is costing us several million euros.
"We are in the middle of final negotiations with potential investors. Stable operations are a prerequisite for the success of these negotiations.
"That's the only way we can secure as many jobs as possible," he added.
'Fear and anger'
The airline has been struggling for some time and over the past two years has announced losses amounting to 1.2bn euros (£1.1bn).
In an internal memo, chief operations officer Oliver Iffert said: "We must return to stable operations. That is crucial in order to bring talks with investors to a successful conclusion.
"Today is a day that threatens the existence of Air Berlin."
Germany's giant services sector union Verdi expressed solidarity with the absent pilots and warned that more workers could call in sick.
Verdi board member Christine Behle said: "All the conversations surrounding insolvent Air Berlin are always about its economic interests, never about the jobs of its more than 8,000 employees.
"The fear and anger among Air Berlin staff is escalating because the future of whole families are at stake."
The German government has agreed to provide a bridging loan of 150m euros to keep the airline flying for three months during the busy summer season.