Children surfing the internet are being targeted by thousands of websites encouraging them to develop potentially fatal eating disorders. They display pictures of ultra-slim women along with tips on how to starve for days while convincing your parents that you are still eating.

Campaigners compare them to prosuicide sites and are calling for them to be shut down.

The video-sharing site YouTube hosts more than 2,500 such examples while the social networking sites such as Facebook, MySpace and Bebo have hundreds more.

Users boast that eating disorders such as anorexia and bulimia are a "lifestyle choice" and not diseases. Videos with titles including "40 reasons not to eat" and "how to be bulimic" are all too easily accessible by the website's millions of users.

One, entitled "thinspiration", has attracted 112,000 hits and carries messages from users including: "You will be FAT if you eat today. Just put it off one more day."

There are also messages from adolescent girls desperate to emulate the figures of stick-thin celebrities and models.

One has written starvation tips including: "Feel your bones. Just wrap your fingers around your wrists or along your collarbone. This will help you realise that you're making progress and will also deter you from eating because you don't want to lose those bones, do you?"

She encourages others to look at the websites frequently to keep themselves motivated, adding: "Do you think you'll want to eat after reading trigger-happy quotes and looking at skinny models?"

The number of victims of eating disorders has tripled in the last decade in Britain, and adolescent girls are statistically most at risk. The Royal College of Psychiatrists estimate that 1 per cent of 16 to 18-year-olds suffer from anorexia.

Last week the Home Office published plans to make social networking sites display adverts for the emergency services to encourage children to call police if they feel they are being targeted by abusers, and to direct users towards expert help and advice.

The guidelines also warn parents about pro-anorexia and prosuicide groups, but campaigners said they did not go far enough and called for the groups' web pages to be removed.

A spokesman for B-eat, an eating disorder support group, said: "Thinspiration is designed to promote the anorexic lifestyle. It is not there as a warning - it is there to encourage.

"Webmasters should take a harder line on what is allowed on these websites. They have acted to remove other content that is seen as dangerous.

"Anorexia and bulimia are dangerous activities and people can be encouraged to copy them, so they should be treated in the same way."

A spokesman for the Government's Child Exploitation and Online Protection centre said the groups were not illegal and could be dealt with only by the website owners.

"It is a grey area which makes the groups technically legal, despite being on a par with prosuicide sites. What we would say is that sites like YouTube should assess and decide whether these videos are within their terms of use."

Vathani Navasothy, founder of the eating-disorder therapy group Awakening Dawn, said: "There is no grey area. By allowing this the websites are advocating it. It is like advocating the spread of coronary heart disease."

MySpace, Bebo and Facebook have previously defended their decisions not to censor proanorexic pages. They have said they work with eating disorder organisations and carry adverts from the National Eating Disorder Association to target sufferers.

A YouTube spokesman said the site would remove videos only if other users flagged them up for review.

"YouTube is a community site used by millions in very positive ways. Sadly as with any form of communication, there is a tiny minority who try to break the rules. On YouTube, these rules prohibit content like pornography or gratuitous violence.

"When people see content that they think is inappropriate they can flag it and our staff then review it. If the content breaks our terms then we remove it, and if a user repeatedly breaks the rules we disable their account."