Charities involved with eating disorders have called for tighter controls on the internet after it emerged that popular social networking sites such as MySpace and YouTube were being used to promote anorexia.

Pro-anorexia websites, on which girls exchange extreme dieting tips and view “thinspiration” videos featuring alarmingly thin women, have existed for some years. But they have always been difficult to find and the people posting on them have remained anonyomous.

Now pictures and footage of underweight teenagers are emerging on more mainstream sites, reaching a potential audience of tens of millions.

On Facebook, some groups extol the virtues of anorexia as a lifestyle choice. MySpace’s groups include one that has more than 1,000 members. Its rules state: “No people trying to recover. It ruins our motivation.”

Thousands of people have viewed film clips of emaciated looking teens and twentysomething women on YouTube which, along with the other networking sites, has rules against posting harmful content. The two to ten-minute videos often feature the more slender celebrities such as Victoria Beckham and Kate Moss, neither of whom is anorexic. They also show images of underweight women in their underwear.

Eating disorder charities have called on social websites to look closely at their online material. Susan Ringwood, chief executive of the charity Beat, said: “Pro-anorexic sites weren’t easy to find and most responsible internet providers would cut them out. But on the networking sites there isn’t the same control over them at the moment. Some of the more hardcore struff does seem to be getting on to these sites. We are concerned that this is a trend.”

However, support groups claim that making and discussing videos are the only forms of help available to some young women who are afraid to talk openly about their concerns.

Kay Fielding, 16, from Gloucestershire, aspires to weigh less than 7½ stone. The 8st sixth-former said: “I’m unhappy with my weight and the way my body looks. I wish I could be slightly slimmer because I just feel there are some parts of my body weight I cannot shift.

“My favourite celebrity and model is Kate Moss and my next goal weight is her weight of 107lb, which I read somewhere. I first started looking at thinspiration videos about a year ago. I felt like a failure because I couldn’t fit the image they projected.

“But I was also really inspired because the people who made the videos were only trying to support other people with eating disorders, not trying to force a condition on someone else. I look at the videos now every few days and I save the images I like on my computer. I restrict my calorie intake, and if there’s an event that’s coming up I might fast for a day, then eat just fruit and vegetables. I’m not dangerously skinny, but I know my mum does worry.”

Some of the videos carry a warning that they might trigger eating disorders or are marked as suitable only for adults, which means web users have to register as an 18-year-old to see them.

One 22-year-old video-maker, calling herself Lolaleberg, insists that she does not promote anorexia or encourage girls to become anorexic. Lolaleberg, from Berkshire, said: “I make hamshire, said that thinspiration videos did not help people wanting to recover from an eating disorder and did not promote good health.

Deanne Jade, principal of the National Centre for Eating Disorders, said: “I have no firm view that YouTube should ban them, they only pop up again in a different guise.

“There is no proof that they cause anorexia and although many are firmly helping helping people to stay anorexic, they also support people who decide to get well.”

A YouTube spokeswoman said that it did not comment on individual videos. MySpace said: “Rather than censor these groups, we are working to create partnerships with organisations that provide resources and advice to people suffering from such problems. We will target those groups with messages of support.”