Lufthansa canceled half of Tuesday’s scheduled flights after a public-sector union set plans for a strike that will stifle operations at the German airline’s two largest airport hubs.

More than 800 out of 1,600 trips won’t take place on Tuesday, the Deutsche Lufthansa AG unit said Monday. The Ver.di union said it will stage a warning strike of ground staff and fire fighters starting at 5 a.m., taking place at airports in Frankfurt and Munich, as well as smaller sites at Cologne and Bremen.

The strike is “completely unacceptable,” said Deutsche Lufthansa AG’s personnel and legal chief, Bettina Volkens, noting that the company isn’t a negotiating party in the conflict. The union is fighting for 6 percent higher pay for 2.3 million public-service workers.

About 58 of the canceled flights are long distance, Lufthansa said. Altogether, about 90,000 passengers will be affected. Customers scheduled to fly through Frankfurt or Munich on Tuesday will be allowed to postpone their booking free of charge within seven days, while flights that are operating may be delayed.

Customers flying domestically can use Deutsche Bahn rail instead, and air travel should be back to normal on Wednesday, Lufthansa said.

Lufthansa rose 0.4 percent to 26.45 euros as of 3:31 p.m. in Frankfurt. The shares are down 14 percent this year.