Teenage girls are becoming involved in dangerous games of competitive dieting thanks to the proliferation of pro-anorexia websites.

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.

“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 said. “When I started this research last January I came across a website set up by a girl who was disgusted with herself because she had put on a few pounds at Christmas. 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” of blogs by individuals on sites such as YouTube and Tumblr, many featuring sexualised images of scantily clad girls, Dr Bond said.

“Girls are posting images of themselves and are then approached by people who want to sell them on porn sites because there is a market for ‘skinny porn’. But the girls are unaware they are being looked at for sexual gratification.

“Some of the websites are works of art in themselves, very beautiful with illustrations, clips of film and letters and poems to ‘Ana’. To a vulnerable teenager they appear lovely, pretty and attractive and give a sense of belonging. But they have a gruesome side too.”

Many focus on purging, starving and the use of laxatives and diet pills which cannot be obtained in the UK but are available on the internet. Some include advice on self-harming and have links to pro-suicide sites.

A disturbing feature of anorexia is the rivalry that can occur between “ana-buddies” who meet on the websites and vie with each other to starve themselves to the point where their lives may be in danger.

Anorexia usually begins in adolescence, affecting 1 to 2 per cent of teenagers and university students, though it can occur at any age. Dr Bond said the threat posed by the sites should be tackled through a combination of education and better policing. “Eating disorders are not going away, if anything they are becoming more common,” she said.