EU consumer commissioner Meglena Kuneva said the results of an EU-wide investigation against misleading advertising and unfair practices on airline ticket selling websites showed that over 50 per cent of those studied had "irregularities".
The commission said its investigation covered Europe's leading airlines and low-cost carriers, as well as other websites selling airline tickets.
It said the results of the inquiry show that over 50 per cent of all websites showed irregularities, in particular relating to price indications, contract terms and clarity of proposed conditions.
It said that, in the week of September 24th-28th, national enforcement authorities in Austria, Belgium, Bulgaria, Cyprus, Denmark, Estonia, Finland, France, Greece, Italy, Lithuania, Malta, Portugal, Spain, Sweden and also Norway scrutinised 447 websites to check their compliance with EU consumer law.
Of the 447 websites, 226 were found to have irregularities.
The inquiry found 46 out of 48 Belgian websites to have irregularities, as well as 9 of 11 Italian sites and 20 of 30 Finnish sites. Ryanair was among those named.
No Austrian, Cypriot or Greek sites were found to have irregularities.
The commission said this sweep check - which focused on clear pricing, availability of offers and fair contract terms - will be followed by an enforcement phase when companies are contacted by authorities and asked to correct websites or clarify their position.
Ms Kuneva said she is giving companies four months to respond, after which they will face possible legal action, and will be 'named and shamed' if they fail to take action to bring their websites in line with EU law.