Bargain hunters beware: More than half of Web sites selling airline tickets in the European Union have catches or small print which leave passengers with more to pay than they thought they had bargained for, the EU said Wednesday.

“There is a substantial problem in the industry,” said EU Consumer Commissioner Meglena Kuneva after investigators checked more than 400 Web sites through much of the 27-nation bloc. “Over 50 percent of sites checked seem to mislead consumers.”

Allegedly cheap tickets are often leaden with hefty charges. Kuneva said investigators found offers for 20 euros only to find out five pages further on that the total price was over 100 euros.

Sometimes there simply are no flights for the lowest prices advertised or insurance is automatically added to the price.

Belgium came out worst, with 46 of the 48 sites showing irregularities. Of the 15 EU nations checked, only Austria, Cyprus and Greece had a clean bill of health in the test of airline and travel Web sites.

Kuneva said Web site operators had until the end of January to clean up their act or face fines or closure.

EU airline travel accounts for some 700 million passengers a year and it is one of the areas with most consumer complaints. In all, 447 sites were checked and 226 showed irregularities. Misleading pricing was the most prevalent problem.

Jim Murray, the head of the BEUC consumer advocacy group welcomed the initiative and hoped “it will be the first of many” to counter illegal practices affecting consumers across Europe.