British American Tobacco and three other e-cigarette firms have been told to stop promoting their products on public Instagram pages.
Health campaigners described the move as a "major step forward" in preventing teenagers and children trying out and becoming hooked on vaping.
However, the four firms - British American Tobacco, Ama Cape Lab, Attitude Vapes and Mylo Vapes - all argue their Instagram posts provide factual information such as the name, content and price of their products.
The Advertising Standards Authority disagreed with their argument and said they "clearly went beyond the provision of factual information" and were promotional in nature.
Its ruling followed pressure from a number of anti-smoking campaigners, who said the firms were breaking advertising rules by promoting unlicensed products which contained nicotine on social media platforms.
The complaints focused on seven Instagram posts earlier in 2019 for BAT's e-cigarette brand Vype, three of which featured images of singer Lily Allen.
Others had images of models puffing on a vape pen and a message congratulating actor Rami Malek on his Bafta for best actor in Bohemian Rhapsody.
BAT argued it used Instagram to communicate "factual information" about Vype that adult vapers and smokers might not be aware of, nor able to find on its e-commerce website.
A post by Mylo Vape in October included a woman with an e-cigarette captioned "#repost @rae-elanor loving her #mylo".
Attitude Vapes posted an image of someone smoking a vape pen captioned "Attitude is everything #LiquidsWithAttitude."
In March, Ama Vape posted an image of someone smoking its Shock Spearmint product with the caption: "SHOCK Spearmint! With @amilegiffen."
The ASA said a social media page or account might fall within the rules if it could only be found by those actively seeking it.
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However it noted it was possible for posts from a public Instagram account to be automatically distributed to users following the hashtags they contained or possible for the posts to appear on another user's Instagram Explore page, where content was automatically generated for the user to see.
The ASA said: "We considered both mechanisms were consistent with content being pushed to consumers without having opted into to receive the message it contained and therefore that neither mechanism was equivalent to actively seeking out information about e-cigarettes.
"Given those characteristics, we considered that material from a public Instagram account was not analogous to a retailer's own website and that material posted from such an account was therefore subject to the prohibition on advertising of unlicensed, nicotine-containing e-cigarettes, meaning that neither promotional nor factual content was permitted."
The ASA also found that, in any case, the ads contained content which "clearly went beyond the provision of factual information and was promotional in nature".
Professor Anna Gilmore, Director of the Bath University Tobacco Control Research Group, said: "This is a major step forward in stopping the tobacco industry from promoting its new addictive products to children and teenagers.
"But given that cigarette sales are falling and tobacco companies are desperate to recruit young people into using these new products, ongoing vigilance is essential."
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