Boots, Superdrug and Holland & Barrett have broken their own policies by selling diet pills to a 17-year-old without checking for ID, the BBC has found.
The teenage actress, sent by BBC Watchdog, was able to buy diet pills in 17 out of 18 stores visited.
A single branch of Boots was the only store to deny the undercover actress the sale because she didn't have ID.
When presented with the findings all the retailers promised to take action.
In all, the teenager visited six different branches of Boots, Superdrug and Holland & Barrett.
In both Holland & Barrett and Superdrug every store sold the actress the pills.
Several of the Superdrug and Holland & Barrett stores attempted to sign her up to their loyalty card scheme.
It is not illegal to sell diet pills to young people - but Holland & Barrett, Superdrug and Boots all have policies in place which are supposed to make sure that they are not sold to anyone underage.
In addition, most of the products are clearly labelled with recommended age restrictions.
Dr Anna Colton, a child psychologist, described Watchdog Live's results as "absolutely terrifying".
She added: "You are better off having no recommendation than having a recommendation everyone ignores because it gives such a false sense of safety.
"I think staff should be trained that if someone comes in as they are with an energy drink or with alcohol, if someone comes in who looks underage, you ask for ID and then you say, 'I'm really sorry, no.'"
In response to Watchdog's findings, Boots said it had a number of products and services to help customers lose weight in a "responsible way". It also said it had pharmacists and trained staff who were able to give advice on using diet aids safely.
"These products are regulated by the Medicines and Healthcare Products Regulatory Agency (MHRA) and Boots follows all the relevant guidance for their sale.
"On our website and on the product boxes it clearly states the recommended age guidance for the products in question.
"In addition to following the correct guidance as to how these products are currently sold in store, we are looking at how our colleagues communicate with customers to best meet their needs when buying these products."
Holland & Barrett said ensuring its products were sold responsibly was of the "upmost importance" to it.
It said it was "very disappointed" that the investigation had highlighted cases where it had "fallen short of the standards we expect in our stores, and are taking immediate actions to ensure this is addressed".
"We are ensuring information around age restrictions is strengthened in our stores and online, and are reviewing our training to ensure all colleagues are clear on our policies relating to these products.
"Importantly, our colleagues will now be prompted to request ID for customers who look under 25 when purchasing all age-restricted weight management products, and we will decline the sale of these products to customers who cannot demonstrate they are above 18."
Superdrug said it wanted to reassure its customers that "immediate action" had been taken as a result of Watchdog's investigation.
"Our actions consist of checking that all appetite suppressant products have a till restriction. When scanned through the till, it activates a 'prompt' to flash up and remind cashiers that the product is not to be sold to those under the age of 18 years and to ask for photographic ID.
"We have also issued out specific training on age restricted diet products to all staff.
"In addition, only registered customers, who've confirmed their date of birth, can buy age-restricted products online. Furthermore, we want to help educate customers about these age restrictions and have plans to implement shelf signs in all stores that sell these diet products informing customers of our policy."
Presenter Nikki Fox met Katie who is now 21 - she told the programme that at the age of 14 she was able to go into stores in her school uniform and buy diet pills without getting ID'd. Katie told the programme "no-one questioned me, no one ID'd me".
Stephen Powis, the National Medical Director of NHS England, said: "We know that these diet pills can cause physical problems on occasions such as abdominal pain or diarrhoea. But we're particularly concerned about potential effects on mental health.
"That there is a lot of pressure on young people today and many of them are particularly concerned by body image, about how they look and that potentially leading to an epidemic of mental health.
"We need retailers to ensure that they are not selling products to those that are vulnerable, particularly young people who are under the age of 18."
You can watch the full investigation on Wednesday 15 May at 20:00 BST on BBC One or on iPlayer afterwards.