New generation phones have colour screens and video

Orange, O2, T-Mobile, Vodafone, Virgin and 3 have signed up to a Code of Practice, which imposes an "18" classification on adult content.

E-commerce Minister Stephen Timms said it was a much-needed "excellent example of responsible self-regulation".

It covers images, video, gambling, games, chatrooms and net access but not premium rate voice and SMS services.

They will still be regulated by the ICSTIS (Independent Committee for the Supervision of Standards of Telephone Information Services) Code of Practice.

Age assurance

Mobile operators have been working on a framework to control and censor the distribution of mobile content over the last 12 months.

MAIN POINTS OF THE CODE Commercial content unsuitable for under 18s classified "18" 18 content not available until networks satisfied customer is at least 18 Classification framework in line with other media Chatrooms available for under-18s to be moderated Parents/carers able to use filters to operator's net services to restrict content Operators to work with law enforcement agencies to report content that may break criminal law Operators to combat bulk and nuisance SMS Operators to give advise on nature and use of mobile devices and services Code to be available on operators' websites Source: T-Mobile

It means content with an 18 certificate will only be available when the network operators verify the age of the user.

An independent body will decide on standards for the classification system and will work in a similar way to other parts of the media industry.

Children's charities welcomed the step, but said they hoped those operating similar services through fixed-line net would do the same.

"These new measures are a great step forward for the mobile phone industry and we very much welcome them," said John Carr, net advisor for NCH.

"The new code is going to make many people ask why, if the mobile people can do it, the fixed internet people can't.

"We are calling on them to take this forward as a matter of urgency," he said.

Wap worry

With newer 2.5G and 3G mobiles that have colour screens, video and mobile surfing, there has been a growing concern about how much inappropriate content children have access to from their pockets.

Chatroom access through mobiles have been a particular concern, as they are popular with many children and teenagers.

The code will ban those under 18 from mobile chatrooms which are not moderated, in an attempt to protect children from online grooming.

Mobile chatrooms - or Wap chatrooms - are similar to net chatrooms, but involve groups of people sending messages which all those in the same room can read on their phone screens.

Mr Carr said children's charities would be closely monitoring how the guidelines worked in practice and reviewing its effectiveness.