German airline Lufthansa said 113,000 passengers would be hit by flight cancellations on Monday when cabin crew staff will strike over cost cuts.
It said 929 flights from Frankfurt, Duesseldorf and Munich will be cancelled as the dispute moves into its third full day.
Members of the UFO union are planning further strike action throughout the week at further airports.
Lufthansa and UFO are in a long-running dispute over retirement benefits.
Monday's strike will run from 03:30 GMT until 22:00 GMT in Frankfurt and Duesseldorf and 23:00 GMT at Munich.
It will affect more than three times as many flights as a nine-hour walkout last Friday, and almost double the 520 cancelled on Saturday.
Flights were reported to be running normally on Sunday.
Lufthansa said two thirds of the group's 3,000 flights would still run on Monday, as subsidiaries Austrian Airlines, Germanwings, Eurowings, Brussels Airlines, Air Dolomiti, Swiss and Cityline would not be affected.
Calling the strike "unprecedented in the history of Lufthansa", the company said its board would meet to discuss the consequences.
UFO, which is looking to keep early retirement benefits and pensions for its members, said it is seeking a "clear sign" of a sustainable agreement.
Lufthansa's reluctance to meet its demands is "not comprehensible" given the company just raised its full year profit forecast to €1.95bn, the union said.