Industry regulator PhonepayPlus has unveiled plans to investigate UK mobile phone premium-rate digital services, including ring-tones, downloads and games.

The watchdog highlighted a 40 per cent increase in the number of customer complaints surrounding unexpectedly high bills for downloading digital content to their mobile phones since the beginning of the year.

PhonepayPlus, previously known as Icstis, claims to have received more than 4,500 complaints about mobile content in the first three months of 2008.

The investigation will examine the marketing of digital content for mobile phones, which has already become a £350m a year premium-rate industry.

In a recent adjudication a service provider received a £7,000 fine after placing an advertisement entitled 'Free 4U' for downloads costing up to £4.50 each in a children's magazine.

"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 probe, expected be published in July, will also address unsolicited text message promotions and ring-tone subscription services.

PhonepayPlus has the power to fine service providers up to £250,000 for flouting industry guidelines, and has already dished out £291,500 in penalties this year.