Bulimia is one of the eating disorders promoted on the internet
The Royal College of Psychiatrists (RC Psych) is calling for urgent action to protect vulnerable young people from eating-disorder websites.
It says the number of websites promoting eating disorders has soared with the growth of social networking.
The RC Psych wants the government's Child Internet Safety Council (UKCCIS) to mark such sites as harmful and raise awareness among parents and teachers.
Eating disorder charities say banning the sites will not cut eating problems.
The Royal College of Psychiatrists says the government must do more to address the dangers of pro-eating disorder websites and keep young people safe online.
It has called on UKCCIS, which was established a year ago, to expand its definition of harmful web content to include what it calls pro-ana (anorexia) and pro-mia (bulimia) websites.
It also wants UKCCIS to specifically address pro-eating disorder websites in its plans to raise awareness of e-safety among parents and teachers.
Professor Ulrike Schmidt, chair of the RC Psych's eating disorders section, said: "Pro-ana and pro-mia websites advocate anorexia nervosa or bulimia nervosa as a lifestyle choice, rather than as serious mental disorders.
"Research shows that, even for healthy young women, viewing such websites induces low mood, low self-esteem and increased body dissatisfaction.
"The broader societal context in which pro-ana and pro-mia sites thrive is one where young women are constantly bombarded with toxic images of supposed female perfection that are impossible to achieve, make women feel bad about themselves and significantly increase their risk of eating disorders."
Susan Ringwood of the eating disorder charity, Beat, welcomed the RC Psych's position on pro-anorexia websites.
But she said: "We know that banning sites will not deal with the root causes of this challenging and misunderstood condition."
UKCCIS was launched by the government in September 2008.
One of its key tasks is to develop a strategy for Child Internet Safety.
A UKCCIS spokesman said: "We are working hard to ensure that vulnerable young people are signposted towards places where they can get help and support when accessing websites such as these.
"And we are working closely with charities and industry to create appropriate and intelligent moderation practices in this sensitive area.
"Banning discussions on issues like anorexia from sites that are popular with young people risks driving them away from popular sites, into parts of the internet where there is nothing to put the discussion in context.
"The Byron Review did address issues around vulnerable groups and specifically internet sites that promote eating disorders.
"Professor Byron made a number of recommendations in this area - all of which have been accepted by government - and are being taken forward by the UK Council for Child Internet Safety."