The advertising watchdog has criticised mobile phone network 3 for price comparison adverts over the summer, branding them "misleading".

Britain's newest mobile phone network has attracted more than 400,000 customers to its ThreePay pay-as-you-go service using a combination of aggressive price-based advertising and large bundles of minutes and texts.

O2 and T-Mobile complained to the Advertising Standards Authority about the comparisons used. The ASA upheld all but one of their nine complaints, saying that the information in the ads was misleading because it failed to explain exactly how comparisons were calculated.

The ASA looked at a series of national press adverts in which 3 claimed: "It's only 5p a minute to call any other mobile network" and asked "How much is O2 charging for pay-as-you-go? A 10p per minute call to any other mobile network".

Both adverts were based on customers buying a £25 per month VideoTalk voucher from 3 which gives 500 minutes of calling but has to be used within 30 days.

The ASA ruled that the rate was 5p only if a consumer used all their 500 minutes. It said the adverts "misleadingly implied the call cost per minute was 5p for ThreePay regardless of whether the full voucher was used".

The ASA also rapped 3 for a series of adverts claiming it provided texts at 5p each, saying that again that price was based on a customer using up their entire allowance.

In a blow to O2, the ASA "acknowledged that consumers who used a full £15 of ThreePay's VideoText voucher in a month would get more voice call minutes to landlines, calls to other UK mobile networks and to voicemail than with the complete use of two £10 vouchers from O2".