The issue relates to consumers who try to book cars online. Depending on their IP address some car rental companies reroutes them to different web pages where more expensive prices are on offer.
The European Commission has written to at least six car rental firms operating across Europe, including Ireland, urging them to stop discriminatory practices
The practice of using online booking systems which 'automatically re-route' customers to other web pages results in customers not being able to access the lowest fares on offer.
The Commission says it is aware of one case where a customer from Germany was asked to pay double the original price on offer when attempting to rent a car in the UK after entering his country of residence.
The Commission says it has received hundreds of complaints against the six companies - identified as Europcar, Hertz, Sixt, Enterprise, Goldcar, and Avis - and wrote to the CEOs of the companies on July 23.
The six companies were warned that such practices run contrary to the notion of a single EU market, the Commission said.