Brussels airport was closed on Tuesday and the nearby airport in Charleroi only accepted landing aircraft with no planes taking off because of industrial action by Belgian air traffic controllers, officials said.
The airport has been running at limited capacity since reopening on April 3 following an Islamic State suicide bombing on March 22.
"Currently no air traffic control is possible at Brussels Airport," Brussels Airport said in a statement on its website.
"This means that there are no inbound or outbound flights. The situation is being assessed and the management of Belgocontrol is looking for operational solutions."
Belgocontrol, which handles Belgian airspace, said it was seeking a solution with the trade union.