The EU Consumer Affairs Commissioner has criticised hundreds of websites for hiding the true cost of downloads and failing to tell customers they were signing up for subscriptions.

Meglena Kuneva said that the crackdown on online sellers of mobile phone ring-tones will focus on 80 per cent of 558 EU-wide websites investigated for employing suspicious business practices.

"Far too many people are falling victim to costly surprises from mysterious charges, fees and ring-tone subscriptions that they learn about for the first time when they see their mobile phone bill," she said.

Kuneva explained that rogue traders of mobile content will be targeted by national governments.

Often aimed at teenagers, ring-tones comprise nearly a third of mobile content in Europe, accounting for €691m in sales revenue in 2007.

"We need to get a clear message out, particularly to teenagers and children, to be on your guard," 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."

Kuneva also warned of further clandestine investigations in specific markets in the coming months, but refused to reveal details.