Thousands of young people with eating disorders are being preyed on by “anorexia coaches” operating on an anonymous messaging app, a Guardian investigation can reveal.

Kik, a popular app among children and teenagers that has previously come under fire over safeguarding issues, hosts dozen of pro-anorexia group chats that are open to the public.

Within minutes of joining a conversation, users can be approached by an “anorexia coach” who calls on vulnerable young people, usually girls, to message them for weight-loss advice. If the recipients do not follow strict advice to limit their calorie intake, the coaches tell them they need to be punished – in some cases asking them to send nude pictures.

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The Guardian found at least 71 pro-anorexia groups with a combined total of more than 2,000 members operating under easily searchable terms. On joining the groups, people are asked to give their age, height, current weight and goal weight.

This comes despite community guidelines for the app saying self-harm, eating disorders or suicide should not be shown or promoted.

Dr Louise Theodosiou, of the Royal College of Psychiatrists, described the findings as “extremely concerning”.

“You think about safeguarding and exploitation … One of the things you’re thinking about a lot is anonymity and transparency and these girls are giving a lot of information about themselves but [there is a] lack of transparency about who these men are,” she said.

“It’s a different phenomenon from the ‘thinspiration’ and ‘pro-ana’ phenomenon … this [is a] potentially very exploitative situation.”

Luciana Berger, the MP for Liverpool Wavertree, said: “It is incredibly disturbing that users of Kik are able to exploit and manipulate vulnerable young people in this way.

“The government’s announcement that they intend to introduce new legislation to compel social media companies to remove content that encourages suicide and self-harm earlier this month is welcome, but well overdue. It also does not go far enough.”

The Guardian revealed this month that there has been a big rise in hospital admissions for eating disorders in the past year, prompting concern from experts about a growing crisis of young people experiencing anorexia nervosa and bulimia nervosa.

Figures show year-on-year rises in hospital visits, with admission numbers more than doubling from 7,260 in 2010-11 to 16,023 in the year to April 2018. The latest figure is up from 13,885 the year before – the highest rise in eight years.

One woman who has an eating disorder, and worked with an anorexia coach on Kik, said she was asked to send naked pictures when she did not meet her calorie limit for the day. “If you deviate from a diet plan they would demand you send naked pictures of yourself as punishment,” she said.

“Most of the time, if you’re that desperate, which most girls are, they will message [the coaches].”

She added that she felt most of the coaches were men. “They ask for photographs of any food you eat and say you also have to ask permission to do anything, such as exercise.”

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One of the people who appears to be a male coach running a group has been on Kik for more than three years. Another user has been on for more than four months and is an admin of multiple groups.

A main attraction of Kik that differentiates it from other messaging apps is its anonymity. The Canada-based app, which was founded in 2009 and claims 300m accounts worldwide, says users who feel unsafe should report content to them.

“Kik is being used to promote eating disorders and contains pro-anorexia groups despite the platform’s community guidelines which prohibit such activity,” said Jennifer Grygiel, a social media expert and assistant professor of communications at Syracuse University.

The app has come under fire in the past for its approach to extremist groups who have used the platform. Chief constable Simon Bailey, the child protection lead for the National Police Chiefs’ Council (NPCC), said last year that Kik was prone to abuse by paedophiles after the number of UK child abuse reports relating to the service quadrupled in the last two and a half years.

Charities and experts recently called on Instagram to crack down on images of eating disorders in the same way the app was planning to do with graphic self-harm images, as part of a series of changes made in response to the death of the British teenager Molly Russell.

A spokesperson for the eating disorder charity Beat said: “So-called ‘pro-ana’ and ‘pro-mia’ content is widespread on social media and can be very harmful for people suffering from an eating disorder. People will not develop an eating disorder by being exposed to content that glamorises eating disorders, but research shows that such content helps perpetuate the illnesses for people who are already suffering.”

A spokesperson for Kik said: “We take online safety very seriously, and we’re constantly assessing and improving our trust and safety measures … We encourage users to report content that they believe violates the Kik Terms of Service and Community Standards. Users are also able to block other users they no longer wish to chat with, or ignore chats from people that they don’t know.”