Children can access internet porn via mobile phones due to a loophole in phone safeguards, Which? magazine says.
Researchers got codes for porn sites using a teenager's phone
Youngsters are able to obtain codes for adult websites by paying for them using premium rate text messages, it claims.
Which? editor Malcolm Coles said: "There's nothing to stop children getting hold of hardcore pornography through their mobile phone."
Industry regulator Icstis said mobile phone companies and internet providers were responsible for age checks.
But a spokesman added: "The situation shouldn't be allowed to continue."
To support its claims of a loophole, Which? researchers got codes for eight porn sites using an O2 phone registered to a 15-year-old girl.
The filter system used by the phone company blocked adult material being sent directly to the handset, but still let through access codes to websites showing hardcore sex films, the consumer group said.
And it cost £1.50 to pay for the codes that gave access to internet porn sites.
Which? warned that young viewers could view by entering the codes into a computer.
Mr Coles said: "There are supposed to be safeguards to stop this sort of thing, but they're obviously ineffective.
"This loophole needs to be closed as soon as possible."
The consumer group said the number of pay-per-view internet sites accepting payment by premium rate text messages had increased over the past few years.
An O2 spokesman, who stressed that the company took the subject "very seriously", said some adult websites did accept payment via mobile phones.
"We have an age verification process for any commercial content that can be viewed on a mobile phone," he said.
"We also offer a parental control on all of our mobiles that can be activated from the phone."
He added: "While it's the responsibility of those websites to prevent under- 18s from viewing the content on a computer, we do not want our mobiles to be used as a payment mechanism without age verification.
"We are in the process of updating the agreements in place with companies that provide mobile content over our network."
Meanwhile, a spokesman for industry premium rate phone and text message Icstis said: "Obviously this is a loophole. Kids can access adult content on their PC screens and be billed via their mobile phones.
"We are working with the mobile phone networks to look at how this loophole can be blocked."