European air travellers faced mass disruption on Tuesday with around half of all flights at risk of delays following a technical problem at Eurocontrol, the agency in charge of the continent's skies.
Several of the EU's biggest airports, including Amsterdam's Schiphol, warned of problems and advised passengers to check on their flights because of the computer breakdown.
"Today 29,500 flights were expected in the European network. Approximately half of those could have some delay as a result of the system outage," said a statement from Eurocontrol.
The Brussels-based agency, which coordinates European air traffic control operators, said the cause "has been identified and action is underway to return to normal operations" but that that would not happen until "late this evening".
The breakdown comes a day after the Easter holidays when many travellers are on the move around Europe, and as commuters across France faced disruption from a massive rail strike in protest at President Emmanuel Macron's reforms.
"We have never had anything like this before," a Eurocontrol spokesman told AFP.
Brussels airport said departures were limited to 10 flights an hour. The Belgian airport manages 650 flights a day, according to its website.
Several airports across the continent warned of problems, with Schiphol saying that the "system failure" at Eurocontrol could have "possible consequences" for departures.
Helsinki, Prague and Copenhagen airports also said traffic was facing delays.
Eurocontrol said that there had been a "failure of the Enhanced Tactical Flow Management System", which tracks and manages traffic demand across the continent.
"Contingency procedures are being put in place which will have the effect of reducing the capacity of the European network by approximately 10 percent," the agency said in an initial statement.
It added that flight plans filed before 1026 GMT were "lost" and asked airlines to refile them.
It added that air traffic control had not been directly affected and "there are no safety implications arising from this incident".