People wanting to hire cars online are being charged according to their countries of residence, according to the European Commission.
The charges can vary for some even when using the same hire company, the same location and the same vehicle.
The Commission said that price discrimination based on residence broke the rules of the EU's single market.
The Commission first raised concerns in 2011, and recently wrote to six international car rental companies, telling them to review their pricing policies.
It said that it had received an "unsatisfactory" reply from three companies - Europcar, Hertz and Avis. Consequently, it has made the letter public this week and has requested these companies conduct a full review by the end of this month.
Sixt, Enterprise and Goldcar replied to the Commission’s letter and their practices were not deemed anti-competitive.
It did recognise that some companies are abiding by the non-discrimination principle, but its task is to see that the principle is fully applied throughout the industry.
"The single market should be a daily reality, not only for major international companies, but also for consumers of Europe," said Michel Barnier, vice-president of the European Commission.
See also: http://ec.europa.eu/consumers/ecc/consumer_topics/car_rental_en.htm
