Their teenage users call them "thinspiration" – pro-anorexia websites allowing young girls to post pictures of their stick-thin bodies to drive one another into losing dangerous amounts of weight in competitive and sometimes startling ways.
"One recommended that you go to sleep in the cold, with the windows open, because shivering burns calories," recalls Francesca Carrington Birch, who became anorexic aged 12 and bulimic from 17. Now aged 22 and working in residential childcare, she can see that these websites are "really warped, damaged communities". But for their vulnerable target audience, their proliferation is a dangerous sign of encouragement.
Between 400 and 500 websites promoting anorexia and related eating disorders, visited by thousands of young girls a day, have now been identified in the first review to quantify the phenomenon.
They encourage "starving for perfection", feature pictures of celebrities such as Keira Knightley and Victoria Beckham, and promote diets of 400-500 calories a day (compared with a recommended 2,000 for women and 2,500 for men). Coffee, cigarettes and diet pills are the only things to be consumed keenly.
In one year, more than 500,000 people visited the sites, according to one study, and a 2011 EU survey found more than one in five six- to 11-year-olds had been exposed to one or more sites with "harmful content".
Dr Emma Bond, a lecturer at the University Campus Suffolk, Ipswich, who carried out the review with help from the Nominet Trust and the charity b-eat, says the sites were set up by individuals with eating disorders, who in some cases generated a following of almost religious intensity. There was no evidence of commercial involvement.
"It starts with an individual who wants to share their experience and as they get a following they set themselves up as almost Goddess-like," she explains. "I came across a website set up by a girl who was disgusted with herself because she had put on a few pounds over Christmas. She talked about going back to college where she planned to fast for three days and regain control.
"In under two hours, she had 36 followers saying things like, 'You're wonderful, you're an inspiration to me, I'm only fasting because of you'."
In addition to the websites there are "thousands and thousands" of blogs by individuals on sites such as YouTube and Tumblr, many featuring images of scantily clad girls who are unaware they are being looked at for sexual gratification.
"To a vulnerable teenage girl they appear lovely, pretty and attractive," Dr Bond says. "But they have a gruesome side too."
Many focus on purging, starving and the use of laxatives. Some even include advice on self-harming and have links to pro-suicide sites.
A particularly disturbing feature is the rivalry that can occur between "ana-buddies" who meet on the websites and vie with each other to starve themselves.
"The media should be more responsible in not publishing pictures of very thin models and celebrities because young people wish to emulate them," says Dr Bond. "Eating disorders are not going away, if anything they are becoming more common."