http://eur-lex.europa.eu/legal-conte...4R0261&from=EN is the regulation in question (EC 261/2004). Article 8 paragraph 3 provides:
When, in the case where a town, city or region is served by several airports, an operating air carrier offers a passenger a flight to an airport alternative to that for which the booking was made, the operating air carrier shall bear the cost of transferring the passenger from that alternative airport either to that for which the booking was made, or to another close-by destination agreed with the passenger.