Instagram influencer Charlotte Rollin, who suffers from anorexia, said she was "alarmed" a diet company repeatedly asked her to be a brand ambassador late last year.
Charlotte, who was diagnosed with the eating disorder anorexia at age 15, told ITV News she was asked to promote a weight loss shake despite being open about her struggles online.
The 21-year-old said: "My first thought was how irresponsible it was that the company would be selling this kind of product, but also that they hadn't done their research into my history.
"I'm quite open about talking about my mental health and I've suffered with an eating disorder for around six years now.
Instead of taking up the request, she tweeted a very public rejection and hit out at the firm using social media stars to sell weight-loss products.
"I think in the age of influencer marketing and having big celebrities like the Kardashians and people like that, it's very easy for followers to get caught up in what their favourite celebrities are doing.
"And I think that's where people need to respect their position of power and know there are actual people that are going to be seeing these posts and understand the effect that it can have on young people," she added.
Now, the Committees of Advertising Practice (Cap), the body that writes UK advertising rules, has drawn up an online guide for influencers, in collaboration with the Competition and Markets Authority (CMA), to clarify what counts as an advert, and states that any ads must be marked clearly as such.
It states that any posts that results from a brand giving an influencer money, free gifts or any other perk is subject to consumer protection law, and is also subject to UK advertising rules if the brand has control over the content.
It continues that brands and influencers must make sure consumers always know when they are seeing an advertisement, with failure to disclose a commercial relationship leaving both parties at risk of action from the Advertising Standards Authority (ASA).
The ASA said it was responding to a trend of advertisers entering into commercial relationships with social media influencers and online publishers, which had sometimes resulted in a “blurring of the lines” between advertising and editorial content.