By Duncan Walker BBC News Online

The examination by police of computers used by missing 10-year-olds Jessica Chapman and Holly Wells follows years of mounting concern about the dangers the web poses for children.

According to the Internet Crime Forum about five million British children use the internet, giving them access to more than 100,000 chat rooms.

A BBC investigation found that in a chat room on a website run by the National Library for the Blind, for example, someone was posing as popular teenagers' author Jacqueline Wilson and children communicating with "Jacqueline" had posted their home addresses online.

Children's charity NCH says that in the past two years at least 12 men have been jailed in England and Wales for sexually assaulting children they first met in chat rooms.

Safer surfing

But while it warns that "every single child is potentially at risk" it also tells parents they can do a lot to make the internet a safe place for youngsters.

Internet experts say it is also getting easier to trace offenders.

If Jessica and Holly's disappearance is down to someone they met over the web - a possibility not yet publicly aired by police - there is a good chance the computer used would be traced.

'Crucial message'

John Carr, internet consultant for NCH, told BBC News Online that girls aged between 12 and 17 were the main target for sexual predators.

Childnet 'SMART' tips

Secrecy - never reveal name, address, phone number or password Meet people from the internet with your parents Accepting emails or files can be dangerous Remember someone online may by lying Tell your parents if something worries you

He said: "A girl meets someone in a chat room and they cease to be strangers - they feel like they know them."

Mr Carr said parents need to get to know the internet and what they are looking at.

He said it was not enough to ask children to stay offline, adding: "The crucial message to get across is that kids love to chat - they are going to go online."

A spokesman for children's internet charity Childnet International said chat rooms could be set up so easily that government regulation alone will never be enough.

He said: "One of the challenges to most parents is that their children know more about the internet than they do, so it's absolutely imperative that they sit down and talk to their children and find out about what they are doing."

'Tracing'

NCH hopes that software allowing home computers to record all messages sent by email or via chat rooms will be widely available within a few years.

But while that was unlikely to be installed on the computer Holly and Jessica used, experts believe it could still contain vital clues.

Holly and Jessica disappeared on Sunday

Richard Clayton, of the Foundation for Information and Policy Research, said it was difficult to remain anonymous online.

Anyone contacting children could be traced through any e-mails sent, or through records of individual computers kept by chat room hosts and internet service providers.

He said: "Being anonymous when the authorities are looking hard is not easy.

"There's a lot of logging, there's a lot of tracing and the tools to remain anonymous are not easy to use."

'Top priority'

Recent research suggested that one in five children used chat rooms, of whom one in ten had had a face to face meeting with someone they first contacted over the internet.

Only half were aware that they should tell their parents.

To combat the threat the Home Office set up the Internet Taskforce on Child Protection, bringing together the internet industry, child welfare organisations and police.

Establishing a "kitemark" system for child-friendly sites was among its aims, as was a review of existing laws to see how protection for young children could be improved.

The internet industry itself has also become increasingly concerned about the threat of paedophiles.

Yahoo is said to be preparing to appoint an inspector dedicated to investigating chat room abuse.