Pilots at Brussels Airlines will hold a wildcat strike on Saturday whose impact on passengers and flights the airline said it was scrambling to evaluate.
"We regret that the pilot unions are calling for a wildcat strike tomorrow," a Brussels Airlines spokesman said after the 24-hour strike was announced on Friday.
"We call on the unions to join us around the table to continue the exchanges and avoid unnecessary damage to Brussels Airlines," he said.
The spokesman said "we are still investigating the impact" the sudden strike would have, adding "we apologise to all passengers" who will be affected.
The airline declined to make public the labour issue behind the strike.
But one of the three pilots' unions, BBTK, told the Belga news agency that it was over non-wage benefits and resistance by the airline to increase them in line with inflation.
The Belgian flag carrier, owned by Lufthansa and part of the Star Alliance, operates some 39 aircraft and flies to more than 85 destinations.