Social networking sites need to do more to clampdown against proanorexia support groups, campaigners claim.
The Cheadle Royal Hospital, which houses one of the country's largest Eating Disorders Units, has launched a campaign against the "proana" websites.
The charity b-eat is also backing the move, adding vulnerable youngsters are being encouraged to do "dangerous" things to control their weight.
Susan Ringwood, chief executive of b-eat, said: "The dangerous aspects of any proanorexia/bulimia site is that they can encourage people to avoid treatment or gain ideas about how to maintain their disorder.
"The sooner someone gets treatment, the more likely they are to recover. We are not calling for these sites to be banned, but we do believe they should act responsibly."
Former anorexic Natalia Wasiuta, 25, added: "It is not that these sites are teaching girls how to make themselves sick, what they are doing is offering subtle encouragement and tips on weight loss and fasting.
"The effect is that you visit the site and feel like you are getting support from people who understand what you are going through.
"But they are not supporting you at all."
Cheadle Royal Hospital estimates as many as 1.1 million Britons suffer from an eating disorder, with 90% being women, although they say this number is higher as many cases go undiagnosed.
A spokesman for MySpace said it wants to create partnerships with organisations such as b-eat, rather than censor websites.