(BRUSSELS) - The European Commission announced a crackdown Wednesday on airline ticket selling Internet sites after a probe found that hundreds of them were guilty of unfair practices or misleading advertising.

The investigation in September looked at almost 450 websites in 15 EU nations plus Norway, checking whether ticket pricing was clear, if special offers were explained and the terms of ticketing contracts.

"We discovered that about 50 percent of airline ticket selling websites are currently letting Europe's consumers down," EU Consumer Commissioner Meglena Kuneva said in a statement.

"The figures released today show that there is a substantial problem in the industry," she said.

The 226 companies concerned have been given four months to correct their websites or clarify their positions.

"Once the January deadline expires I will not hesitate to go further and name and shame companies which are still breaking the law," Kuneva said, also raising the prospect of legal action.

Under EU rules, a clear indication of the total ticket price, including taxes and booking fees, should be given in the "headline" price first shown.

Limits on the number of seats available for special offers should also be advertised.

Finally contract terms must be clearly shown, easily accessible and fair as well as made in the language of the consumer.

Unfair practices include mandatory insurance attached to an offer, or where consumers have to explicitly opt-out of an insurance clause rather than opt-in, according to the Commission.

Nearly all the websites examined in Belgium -- 46 of 48 -- had irregularities, while at the other end of the scale, no problems were found in Austria, Cyprus and Greece.

The countries checked were Austria, Belgium, Bulgaria, Cyprus, Denmark, Estonia, Finland, France, Greece, Italy, Lithuania, Malta, Norway, Portugal, Spain and Sweden.

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