The European Commission said it has written to 18 European airlines - including Aer Lingus - asking them to explain what it says are price-discrimination practices.
The commission has given the airlines until the end of February 2004 to say whether they charge different prices for identical flights to residents of different EU countries, and if so, why they do.
In addition to Aer Lingus, the other airlines concerned are: Air France, Alitalia, Austrian Airlines, British Midlands, British Airways, Finnair, Iberia, KLM Lufthansa, Luxair, Meridiana, Olympic Airways, Spanair, SAS, SN Brussels Airlines, TAP and Virgin Atlantic.