All flights in and out of Belgium have been cancelled on Wednesday due to strike action, the country’s air traffic control has announced.
The cancellation began at 22:00 (21:00 GMT) on Tuesday night and will run for 24 hours.
Air traffic agency Skeyes said that there was “no certainty about the number of staff in a limited number of key posts”.
The only option left was to not allow air traffic, it said.
A national strike has been called by three major trade union federations.
The agency said its responsibility was to guarantee safety, and it could not do so “in view of the great uncertainty about the occupation of some crucial posts” – because workers do not have to declare in advance if they will turn up or not during a strike.
Flight-tracking service FlightRadar24 tweeted an image showing the flight paths of some planes taking a detour around the entire country after the restrictions began.
Some airports had already taken the decision to close, and a spokesman for Brussels South Airport said they had no alternative.
“We know that there will be substantial road blocks around the airport,” he said. “And, on another hand, we know that the air traffic would be affected.”
He said strike action “has never had such a significant impact”.
Above 7,500m (24,600 ft), flights are co-ordinated with the aid of Europe-wide air traffic control organisation Eurocontrol. Government and military flights will not be affected.