A European Commission investigation has found that four in five websites selling mobile phone services such as ringtones appear to breach consumer protection rule.
The news comes in the same week that the UK's premium-rate phone regulator said that the number of complaints about mobile services is rocketing.
The office of EU Consumer Commissioner Meglena Kuneva has pledged Europe-wide action against rogue traders, some of whom were found to be targeting children and young people.
"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," said Kuneva. "There will be Europe wide enforcement action to track down each of these traders."
A co-ordinated series of checks by EU member states found that half of the 500 sites looked at targeted children, using cartoon characters and TV characters to attract them.
Almost half of the sites checked had irregularities relating to the price of services, and many subscriptions involving multiple cost-inducing events did not ever mention the word subscription.
The people behind most of the websites checked were hard to identify. More than 70% of the sites checked lacked information needed to contact the trader behind the site.
"This is against EU law," said a Commission statement. "The E-commerce Directive requires details of the service provider, including an email address, to be displayed."
"We need to get a clear message out particularly to teenagers and children – be on your guard. It's all about the small print," said Kuneva "There are many reputable traders out there, but to be safe buying these services, check the fine print every time and make sure you are not signing up for more than you bargained for."
The UK's premium rate phone regulator PhonepayPlus (PPP) this week announced that the market for mobile phone premium rate services was mushrooming, and that £460 million was spent in the last year on those services in the UK.
"However, an extraordinary increase in complaints has accompanied this growth. PhonepayPlus received more than 8,000 mobile-related complaints in 2007/8, an 108% increase on the previous year," the regulator said.