One third of customers buying plane tickets online are being ripped off or misled, the European Commission has warned.
A report revealed "serious and persistent" consumer problems involving hundreds of ticket-selling websites throughout the airline industry.
The Commission is halfway through an investigation which has so far triggered action against 137 out of 386 websites checked in a "sweep" search last September, when national authorities simultaneously scanned airline and travel agency websites for misleading advertising.
One in three - 137 out of 386 sites monitored - contained information breaching rules on clear pricing, availability of offers and clear contract terms.
Today's report says that, seven months later, about 50% of those sites have corrected their website information.
Unveiling the findings in Brussels, 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 misled and confused.
"This report shows there are serious and persistent problems with ticket sales throughout the airline industry as a whole. I intend to work with member states to do everything possible to wrap up this investigation by May 1 next year.
"But these findings send a political signal we cannot ignore. I hope operators have now realised the determination of the Commission to act on behalf of consumers where it is necessary."
She warned: "My message to industry is clear - act now or we will act. We will need to see credible evidence of improvement to clean up these sales and marketing practices within the airline sector by May 1 next year or we will be left with no choice but to intervene."
The "sweep" search of websites involved 13 countries, but did not include the UK, which had already conducted its own trawl of airline ticket websites in February 2007.
Last August the UK Office of Fair Trading Office took action against 13 airlines that did not include all fixed, "non-optional" costs, such as taxes, in prices on their websites.
Now the Commissioner is reporting the same main breach of the law across Europe, with today's report showing 58% of website irregularities involved breaking down the total ticket price to give the impression of a cheap fare.
The report says: "For the consumers, the final price to pay when actually booking the ticket is generally higher due to a series of added charges that vary from so-called 'airport charges' to handling fees, booking fees or charges related to credit card payments, priority booking, luggage, fuel, etc."
The other main problem is the lack of availability of seats at the advertised fare, even when the advertised price is accurate.
Such problems blight the whole industry, the report goes on, with the distinction between low-fare and "traditional" airlines increasingly blurred.
The Commission says 79 websites remain under investigation for advertising misleading prices, 21 because of problems of the lack of availability of the advertised offers, and 67 for unfair contract terms - a total of 137 websites in all, because some sites are in breach of more than one of the rules.
The 137 websites under investigation represent about 80 companies - including "large brand" names and "lesser known" companies, none of which the Commission says it can currently name, due to differing legal constraints in some of the member states involved in the inquiries.
The Commissioner said today that enforcement work was now being stepped up, with a final report due in a year: "The Commission will monitor developments in the airline sector over the coming year and assess the need for further action at that time.
"During 2008, updates on enforcement will be posted on national websites which can be accessed via the Commission's consumer affairs website."