The European Commission sent a letter to the CEOs of six car rental companies after receiving several customer complaints concerning allegedly discriminatory practices for renting a car online.

The letter urges the car rental companies (Avis, Hertz, Europcar, Enterprise, Sixt and Goldcar) to stop the practices in various European Union member states. The letter contends that these practices are preventing consumers from getting the best price offered online.

"The single market should be a daily reality not only for major international companies but also for consumers in Europe," said Michel Barnier, vice president of the European Commission — in charge of Internal Market and Services.

The commission's letter points out practices of automatic rerouting — when following the identification of a consumer's internet protocol (IP) address. The IP address may also prevent the consumer from completing any booking online, says the commission.

Alternatively, with no rerouting, the consumer may be given a different price after entering his/her country of residence on the car rental website. In a recent case, a consumer from Germany saw the announced price increase by 100% for renting a car in the U.K. after entering country of residence.

Some car rental companies are complying with the non-discrimination principle, but it is not yet fully applied throughout the sector, says the commission. The commission has deemed that the letter was not followed by effective and comprehensive review of the practices by three out of the six car rental companies contacted (Europcar, Hertz and Avis). Therefore, the commission decided to make the letter public in the interest of consumers.

The commission has asked the car rental companies to report on how they have reviewed their practices by Aug. 30.