Between 400 and 500 websites promoting anorexia and related eating disorders, which are visited by thousands of young girls each day, have been identified in the first review to quantify the phenomenon.
They tell people how to stay thin, promoting diets of 400-500 calories a day (compared with a recommended 2,000 for women and 2,500 for men), backed by coffee, cigarettes and diet pills.
They encourage 'starving for perfection' featuring pictures of celebrities such as Keira Knightley and Victoria Beckham and advocate 'thinspiration' backed by images of thin bodies.
In one year, more than 500,000 people visited the sites, according to one study, and a 2011 EU survey found that more than one in five 6 to 11-year-olds had been exposed to one or more sites with 'harmful content'.
Dr Emma Bond, senior lecturer in childhood and youth studies at the University Campus Suffolk, Ipswich, who carried out the review, said 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.