The OFT has already taken action against 13 airlines in the UK
One in three European consumers buying plane tickets online is being misled, the European Commission has warned.
About half of 137 websites found to have breached EU consumer rules last November have still not corrected the faults, a new report says.
It revealed "serious and persistent" problems, such as breaking down the ticket price to suggest a cheap fare.
The probe did not include the UK, where action was taken against 13 airlines by the Office of Fair Trading (OFT).
The Commission is currently investigatiing the practices of ticket-selling websites across the 27 member countries and Norway. Some of the websites being examined are run by leading airlines.
A sweep of websites last September found 137 out of 386 breached rules on clear pricing, availability of offers or clear contracts.
"It is unacceptable that one in three consumers going to book a plane ticket online is being ripped off or misled and confused," said EU Consumer Commissioner Meglena Kuneva.
"This report shows there are serious and persistent problems with ticket sales throughout the airline industry as a whole."
She added that the industry needed to clean up its act on sales and marketing practices by 1 May 2008 or the Commission would intervene.
The airline industry in the EU caters for more than 700 million passengers a year.
Ms Kuneva said she would be writing to all the member governments and be holding talks with industry representatives.
In August, the OFT told 13 airlines operating in the UK to change its websites after they left taxes, fuel fees and other charges off their advertised prices.
The same breaches are being seen across Europe.
"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 Commission's report says.
The other main problem was the lack of available seats at the advertised fare, even when the advertised price was accurate.