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.

Belgians are facing widespread disruption to public transport on the ground, too. Broadcaster VRT reports that just half of scheduled trains are expected to operate.

The Eurostar services between Brussels and London or Paris are expected to run - but passengers heading to Lille or Calais will face disruption.

Many airlines had already cancelled or rescheduled flights head of some expected disruption during the strike.

The day of national strikes, called by Belgium's three major trade union federations, follows a disagreement over wage growth, which is limited to 0.8% for the next two years.

Unions are calling for an increase in wages, benefits and pensions to what they believe is a reasonable living level, as well as improvements to work-life balance.