Almost nine in 10 e-cigarette shops in the UK are selling vaping products to non-smokers against the industry's code of conduct, an investigation reveals.
The Royal Society for Public Health (RSPH) said 87% of shops were knowingly or unwittingly prepared to sell e-cigarettes to people who have never smoked or vaped.
Experts warn that non-smokers could become addicted if they start vaping.
The RSPH is now urging shops to keep to the voluntary code.
The organisation investigated 100 of the 1,700 specialist vape shops in England, Scotland and Wales.
Just under half of the stores did not check whether the customers had smoked before, and three-quarters of those that did still encouraged non-smokers to start vaping, the RSPH claimed.
What does the code of conduct say?
The Independent British Vape Trade Association represents "all responsible and ethical independent vaping businesses in the UK", both big and small.
It has a 15-point voluntary code of conduct that it asks e-cigarette shops to sign up to.
As well as calling for shops to not knowingly sell their products to under-18s, to avoid claiming health benefits from vaping and adhere to the law, it says the following:
"Vape products are for current or former smokers and existing users of vaping devices, therefore [you should] never knowingly sell to anyone who is not a current or former smoker, or a current vaper."
Footage from the RSPH showed one retailer recommending a vaping liquid with 6mg nicotine content rather than 0mg, despite the customer telling him she did not smoke.
The shopkeeper said: "The nicotine does affect how much of like a hit you get at the back of your throat, so if you want a bit of that, I'd say go for a 6."
Another piece of footage showed a retailer telling a non-smoking customer he would not "want to get you hooked to something" and advising him against buying the product.
Chief executive of the RSPH, Shirley Cramer, said e-cigarettes should be treated as "evidence-based quitting aids, rather than lifestyle products", and therefore should be aimed at smokers.
She said: "We applaud the Independent British Vape Trade Association code of conduct, which is in line with this principle.
"However, as our investigation shows, the majority of vape shops in the UK are not adhering to these important standards.
"We are keen to support the sector to strengthen their codes of best practice, and for individual retailers to sign up and ultimately adhere to them."
Richard Hyslop, chief executive of the Independent British Vape Trade Association, said statistics showed the vast majority of people using e-cigarettes were ex-smokers.
"Vape products should not be marketed to non-smokers or those under the age of 18," he said.
"However, figures produced by organisations such as the Office for National Statistics and Action on Smoking and Health demonstrate that more than 90% of vapers in the UK are adult current or former smokers, therefore we do not believe this to be a significant problem."