Budget airline Ryanair is obliged to be taken to court under EU law if it fails to pay back illegal state aid for flying to Belgium's Charleroi airport.
Ryanair has maintained its services to Charleroi
In February, EU competition officials ordered Ryanair to repay several million euros in subsidies to the southern Belgian Walloon region.
Ryanair has appealed the ruling but in the meantime it still has to pay up.
Now, the Belgian government has told Ryanair it must repay four million euros within a month.
In May, Ryanair lodged an appeal against the EU order to repay some subsidies received for using Charleroi.
Chief executive Michael O'Leary said at the time he would refuse to pay up and filed an appeal to the European Court of First Instance.
But it can take the Court up to two years to make a decision in these type of cases.
The authorities in Charleroi, a region of high unemployment south of Brussels, have been concerned about the economic impact on the local community should Ryanair no longer use the destination.
Ryanair and the Walloon regional government agreed in April to rework the subsidies as part of the airline's appeal to the European Court.