EU regulators said on Wednesday they had opened an in-depth investigation into the financing of Beauvais Airport in France, to determine whether it was in line with EU state aid rules.
The European Commission, which acts as the competition regulator in the 27-member European Union, said in a statement it was also looking into rebates and marketing deals between the airport and its client airlines, to ensure those too met the regulations.
The Commission did not identify the airlines, but those with flights to Beauvais, which is 75 km (46 miles) north of Paris, include Ryanair, Wizz Air and Blue Air.
The airport is owned by the Syndicat Mixte de l'Aeroport de Beauvais Tille.
"The Commission will examine whether agreements between the airport operators and their customer airlines, such as marketing support contracts and discounts on airport and handling charges, have been carried out at market conditions," the statement said.
"The Commission has concerns that such arrangements could give the airlines an undue economic advantage that their competitors do not enjoy."