Consumers are being ripped off, deliberately confused and misled by the websites of leading airlines, a report for the European Commission has found.

A survey of 386 websites in 13 European countries discovered that just under a third were breaking the law, either by concealing charges or misleading the public with promotional offers that were not in fact obtainable.

The European Commission is taking enforcement action against 137 of the websites. Ryanair, Austrian Airlines and the Finnish airline Blue 1 were among the offenders, according to Swedish and Norwegian authorities, while third parties such as tour operators and travel agents also came under fire.

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In total the websites are administered by 80 companies, which have 12 months to demonstrate their compliance with trading regulations or face hefty fines and even closure.

Fifty-eight per cent of them discreetly added extra charges that were not included in advertisements, such as booking fees, "airport charges", or fees relating to credit card payment, priority booking, luggage and fuel. Forty-nine per cent of the illegal websites broke regulations relating to pre-checked boxes on booking forms which required consumers to specifically decline a service in order to avoid extra charges.

The other main problem was the lack of seats available at the fares seen on advertisements.

Meglena Kuneva, the EU's Consumer Commissioner, said that despite repeated warnings, many airlines had refused to improve their online operation. "It is unacceptable that one in three consumers going to book a plane ticket online is being ripped off or misled and confused," she said.