A coordinated investigation across Europe showed that a majority of those websites were in breach of EU consumer rules, the European Union's (EU) executive arm said in a statement.
The inquiry, which was carried out on 558 websites across the 27 EU countries plus Norway and Iceland, found that 80 percent of the sites checked need to be further investigated and many indicated multiple irregularities.
Almost 50 percent of the sites checked are suspected of breaching the rules. On many websites prices and related charges and fees are not clearly indicated or not referred to at all, until the consumer is invoiced via their phone bill.
Half of websites checked targeted children, using cartoon characters or well-known TV characters. Over 70 percent lacked some of the information required to contact the trader.
Other problems include key information hidden in very small print or hard to find on a website or the word "free" used to mislead consumers into long-term contracts.
"Far too many people are falling victim to costly surprises from mysterious charges, fees and ring-tone subscriptions they learn about for the first time when they see their mobile phone bill," the EU Consumer Commissioner Meglena Kuneva said.
"There will be Europe-wide enforcement action to track down each of these traders," she warned.
As a follow-up, companies would be contacted by the national authorities and asked to clarify or correct problems identified.
The Commission said rejection could result in legal action leading to fines or closure of their websites.
Ring-tones alone is estimated to make up 29 percent of the overall "mobile content" market in Europe in 2007, about 10 percent higher than 2006. The value of European ring-tone sales in2007 is estimated at 691 million euros (1.1 billion U.S. dollars).
It is the latest move by the EU to ensure benefits of more than495 million mobile phone subscribers in Europe.
Earlier this week, the Commission said it would present a new regulation in October to cap the charges for sending a text message with mobile phones abroad after it slashed roaming costs for calls and answers by 60 percent last summer.
