Budget airline Ryanair has lodged an appeal against an EU order to repay some subsidies received for using Belgium's Charleroi airport.
Ryanair has maintained its services to Charleroi
The European Commission told the Irish carrier in February to repay millions of euros in subsidies which, it said, amounted to illegal state aid.
Ryanair denounced the ruling, describing it as a setback for competition and cheap fares in Europe.
The firm said it had filed an appeal to the European Court of First Instance.
Last month, Ryanair announced a deal enabling it to continue operations to Charleroi, in spite of the Commission's ruling.
The airline had threatened to cut off its 11 routes to Charleroi, a region of high unemployment south of Brussels.
Ryanair chief executive Michael O'Leary said the EU's decision set "a very damaging precedent for underutilised regional airports all over Europe".
Appeals to the European Court of First Instance can take up to two years to produce a decision.