The European Commission has launched an inquiry into a financial arrangement between Ryanair and the Belgian authorities.
Under investigation is the contract agreed when Ryanair opened its European hub at Charleroi airport in Belgium last April. Under European Union rules State aid to airlines is restricted.
Ryanair has said that there is nothing improper about its arrangement to use Charleroi. It claims that the cost advantages of the airport are available to any airline.
An EU spokesman said the inquiry into Ryanair is "at an early stage".