'As you know, beauty has one name: being thin. Our models are underweight, skinny, thin, bony – just like you. We want you. Regardless of the costs, we want you to join our agency. Let's face facts, on anorexic porn websites, men are masturbating watching your pictures. You are a superstar of starvation and if you were selling and marketing your frame you would be more wealthy than most of us because men would pay any price for watching those pictures."

This was the email Sasha McDonald was sent last year from a pornography agency specialising in anorexic images. McDonald was 15 when she was first diagnosed with anorexia nervosa. "I was very lonely and felt worthless," she says. "I retreated into an online pro-anorexic [pro-ana] community and shared everything. I didn't realise the danger I was putting myself under." Despite receiving professional support, McDonald found herself becoming more entrenched in the online anorexic world. She wrote a blog of her battle with anorexia, recording the small amounts she ate and publishing photographs of herself in her underwear as evidence of her emaciated body.

"I was dangerously underweight and so ill that I felt proud of the comments from other website users saying how beautiful and skinny my body was. I relied on the judgments of the friends I had made on pro-anorexia websites because I assumed they were people like me – scared, depressed, exhausted and battling an illness that torments you continuously," says McDonald.

But McDonald was horrified when a fellow member of one pro-anorexia website emailed her requesting that she join a pornography agency. "My anorexic friend was actually a 46-year-old male with a fetish for skinny women," she says. "He had pretended to be a young girl and persuaded me to share sexually explicit pictures and tried to convince me to join his modelling agency for the super-skinny."

McDonald also found that emaciated photographs she had published of herself on her blog had been posted on anorexia porn forums for users with fetishes about super-skinny women to admire. "Beautiful girl – much prettier than all those meat mountains. Bones and ribs must be very visible. If their BMI [body mass index] is above 15, they are not attractive," says one forum user commenting on a skeletal photograph of McDonald.

McDonald, now 19 and training to be a doctor, had recovered sufficiently to avoid being drawn into the world of anorexia porn, but others with eating disorders have not been so fortunate. Anorexia porn is growing in popularity and the prevalence of pro-anorexia websites is making those with eating disorders easy targets for grooming. Vulnerable users of pro-anorexia websites are increasingly being courted for their emaciated frames by pornography agencies specialising in images of extremely thin women. Optenet, an international IT security company, reported that between 2006 and 2008 the number of pro-ana websites globally increased 470% to more than 1,500 and social networking and blogging has seen a surge in online pro-anorexia content.

One anorexia pornography agency admits to paying owners of pro-anorexia websites for each person who joins it after being contacted via the sites. "I pay the owner of this pro-ana community a donation for every model I found here," confesses a "skinny scout" in the email to McDonald.

The porn agencies' websites have two roles. They sign up new clients and advertise images, films and escort-type services, such as body worshipping, fantasy role play and private photography sessions, for those "hooked on skinnies". Some of this porn is free to access while other "professional" agencies charge a monthly membership fee for regularly updated sexually explicit images and videos of emaciated women. Agencies also host anorexia porn on YouTube and advertise on anorexia pornography forums.

These forums often offer advice on how to groom users of pro-anorexia websites into taking and sharing explicit photographs of themselves.

Anorexia nervosa has the highest mortality rate of all psychiatric illnesses, but in the UK, those with the mental illness have no protection from online exploitation. The legality of anorexia porn means that indecent images of vulnerable adults can be freely published.

"It is government policy that controls a balance between freedom of expression and protection of the public on published material which should be proportionate to the potential harm that might be caused," says Justin Millar, a member of the Home Office's computer crime team. "The general test of obscenity is flexible, reflecting society's attitude towards pornographic material. But even if material is not illegal, it is open to anyone concerned about the content of pornography websites to ask the relevant internet service provider to remove them."

Lucky escape

Samira Jay feels she had a lucky escape. Although she believes that pro-anorexia websites helped her to realise that she had a problem, Jay admits that she would never have published explicit photographs of herself if she had known that they were being used for sexual gratification. Now 19, and studying in Newcastle, she developed anorexia at 11. "It sickens me to the stomach knowing that my photographs could have been used for porn," she says.

"At the time, I was lonely and the pro-anorexia community gave me a purpose in life. Most users posted photographs of their naked bodies because when you have anorexia, the eating disorder is the only thing you feel you are good at, so it gives you a sense of achievement."

Experts agree that much more could be done. "Criminals and predators target vulnerable people online; they simply have no conscience about what they do. Vulnerable people are often not able to make good decisions for themselves and they need others to help protect them," says Jennifer Perry, an e-crime consultant.

"If the UK set the precedent of removing a set of offensive material it would encourage different countries to also take action. Currently, the most effective way to address this problem is education and discussing how people are being approached and exploited."

Many feel that much of the onus should fall on internet service providers. "Responsible internet service providers would remove the most hardcore and toxic material," says Susan Ringwood, chief executive of beat, a UK eating disorder charity. "Pro-ana sites don't hold out any hope at all, they can trap people in a negative cycle of despair."

Jay acknowledges that she is not fully recovered, but she now speaks out about her historic use of pro-anorexia websites to make vulnerable people aware of the dangers of using them. "I knew my photographs might be used as 'thinspiration' by other eating disorder sufferers but the thought that they were being used for sexual enjoyment never even crossed my mind," she says.

Some names have been changed.

• This article was amended on 6 July 2011 to remove a section relating to a case history, pending further research. This article was further amended on 12 September 2011. Following further research, a case history originally included in the article has been permanently removed because of insufficient verification.