Rising complaints have prompted an investigation into the money being charged for premium-rate services for mobile phone users.

The prices for ringtones, downloads and games provided by the £350m-a-year premium-rate industry will now be scrutinised by the watchdog PhonepayPlus; as will companies' prolific use of that most tempting of words, "free".

The move comes after a 40% increase in the number of complaints from consumers, some of whom claim they have been charged for downloads without realising they were incurring costs.

The watchdog received 4500 complaints from January to March, and has now vowed to crack down.

"There are clear rules when it comes to the promotion, content and delivery of mobile phone paid services", said George Kidd, PhonepayPlus chief executive.

"We expect these rules to be followed and any failings of compliance will be punished."

The BBC reports that the regulator handed out fines of more than £360,000 in 2007 and has already imposed penalties of £291,500 this year.

The review will decide whether the regulator has enough power, and the findings are expected to be published in July.