Belgian airspace was to be closed to all flights below 8,000 metres on Wednesday as a national strike threatened to bring the European country to a standstill.
With "no certainty" on how many controllers would be following the strike movement, Belgium's Skeyes air traffic control agency said it was "forced to prohibit" national air traffic between Tuesday, February 12 at 22:00 (2100 GMT) until the same time the following day, a statement said on Tuesday.
No aircraft flying below 8,000 metres altitude -- the area controlled by Skeyes -- will be allowed to fly over the country, Dominique Dehaene, the company's spokesman, told AFP.
Government, military and emergency flights will be allowed, he added.
Above 8,000 metres, a control centre in the Dutch city of Maastricht controls aircraft flying over Belgian territory.
Skeyes was not able to say how many flights or how many passengers would be affected.
A general strike called by three unions, which are calling for higher wages, is expected to paralyse Belgium on Wednesday.
Brussels, home to NATO, will be hosting a meeting of defence ministers that day, with officials from throughout the transatlantic military alliance converging on the city.
"We have no indication of any impact of the strike on the meeting," a NATO official told AFP.
Charleroi airport, the second largest in Belgium and a local hub for low-cost giant Ryanair, had already announced its closure.
Belgium's Brussels Airlines has cancelled all its 222 flights.
The German carrier TUI fly will operate its scheduled Belgian flights from the nearest French and Dutch airports.