Thousands of passengers worldwide have been stranded after an IT fault at Germany's flagship carrier Lufthansa caused flight delays and disruption at airlines across the group.
"There is a group-wide IT system failure," a spokesperson told Reuters on Wednesday.
The company said the problem was caused by damage to several of Deutsche Telekom's glass-fibre cables during construction work in Frankfurt.
Repairs would take until Wednesday afternoon, Lufthansa said.
Photos and videos from several German airports showed chaos, with thousands of passengers waiting to be checked in.
Passengers said on social media the failure had forced the company to organise the boarding of planes with pen and paper and that it was unable to digitally process passengers' luggage.
In a tweet, Lufthansa said: "Currently, the airlines of the Lufthansa Group are affected by an IT outage. This is causing flight delays and cancellations. We regret the inconvenience this is causing our passengers."
Bloomberg News said Lufthansa had grounded all of its flights but the company told Reuters it could not confirm that.
"There are still flights in the air, they will not be brought to the ground," a spokesperson for the company said.
Germany's federal cyber agency BSI was not immediately available for comment.
The IT system failure comes two days before planned strikes at seven German airports that are expected to lead to major disruptions, including potentially at the Munich Security Conference where world leaders are expected to gather.
Scandinavian airline SAS said it was hit by a cyberattack on Tuesday evening and urged customers to refrain from using its app, but later said it had fixed the problem.
Unknown attackers cut cables belonging to Germany's public railway in December in what was seen as the second act of sabotage against Deutsche Bahn in as many months.
Airlines cancelled more than 1300 flights and more than 10,000 were delayed in the United States last month after the breakdown of a key government computer system.
Australian Associated Press