Social media stars could face tougher rules on advertising in posts, the regulator has said.
The Advertising Standards Authority said online ads, such as those on Instragram, should be "obviously recognisable" but that some users with big followings broke the rules.
Such so-called influencers can earn thousands of pounds per post to promote products on Instagram and other sites.
The ASA is seeking evidence on whether ad labelling rules need strengthening.
Under current rules, an influencer must make clear that a post is an advert if they were paid to publish it or received gifts in return.
The ASA can force those who break the rules to take the post down.
However, the watchdog said influencers were still "blurring the lines" between editorial and advertising content, causing "frustration and confusion" among consumers.
This year former reality TV star Stephanie Davis was warned she must clearly label ads on social media following a complaint about one of her Instagram posts.
The Celebrity Big Brother contestant had shared a photo of some vitamins from lifestyle company Convits along with a promotion code.
The ASA ruled it wasn't clear that the post was an advert.
ASA boss Guy Parker said: "Social influencer and native advertising might be relatively new but the advertising rules haven't changed – people shouldn't have to play the detective to work out if they're being advertised to.
"That means the status of a tweet, blog, vlog, Instagram post or story should be clear.
"Our call for evidence will play an important part in helping us understand how consumers recognise and respond to online labelling of ads and how we apply the rules in this area."
The regulator acknowledged many influencers felt confused about "when and how" content should be labelled as advertising.
Currently it recommends posters can include hashtags in posts, such as #ad, but this is not mandatory.
Other recent cases where the advertising watchdog has intervened:
- a post by the make-up blogger Sheikhbeauty on Instagram promoting Flat Tummy Tea that did not make clear she was being paid by the drinks company
- a tweet by the TV presenter AJ Odudu that featured a photo of an Alpro dessert with text describing it as one of her favourite snacks, but without any acknowledgement that she was being paid to promote it on social media
- a video uploaded by Made In Chelsea TV star Millie Mackintosh advertising a Britvic drink that used #sp – referring to "sponsored post" – to identify its nature. The ASA said it did not think consumers would realise what the hashtag referred to
It said it would ask the public about how effective such labels were, and the extent to which "wording, placement, visibility and style" might help people identify an ad.
It also said it would look at what "level" of commercial influence over editorial content people expect to be informed about.
It comes as Instagram itself has launched a new shopping service in the UK.
Shopping in the UK allows businesses to sell directly to their followers.
Users who tap on an item in a post – a jumper for example – would be taken straight to the seller's website.
It follows the introduction of the service in the US last year.
Instagram now has more than 800 million users globally, including 25 million businesses and two million advertisers.
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