One in three European-based airline ticket websites is ripping consumers off, according to the European Commission (EC).

The biggest problem found by the investigation was that six in 10 of the sites looked at gave misleading pricing details. Other breaches of consumer law include misleading advertising and unfair practices such as pre-checked boxes for additional paid-for services such as insurance.

The Commission also said websites often did not give consumers an indication of how many seats were available at the advertised prices, or what conditions were attached to their sale.

Although the Commission is only halfway through this investigation into airline ticket websites, it has already conducted follow-up enforcement action with 137 of the 386 websites it has surveyed. Although it would not name the culprits, it is known that the sites include "large brand" names and "lesser known" companies.

EU Consumer Commissioner Meglena Kuneva said: "It is unacceptable that one in three consumers going to book a plane ticket online is being ripped off or mislead and confused.

"This report shows there are serious and persistent problems with ticket sales throughout the airline industry as a whole."

However, Ms Kuneva said that half of the problem websites have been corrected already because of enforcement actions taken by national consumer protection authorities.

"This is very positive – many companies have responded quickly to contacts from national authorities," she said.

Ms Kuneva said that she would endeavour to have the full report on how the sites were complying with European directives on Misleading Advertising and Unfair Terms in Consumer Contracts by its deadline of 1 May 2009.