YouTube stars have been ordered to make it clear when they are paid to promote products, amid growing criticism they are disguising partnerships with big-name brands.
Fresh guidance by the Advertising Standards Agency (ASA) aimed at social media posts comes after the watchdog found five YouTube videos featuring an "Oreo Lick Race" had misled viewers last year, including one by BBC Radio 1 presenters Dan Howell and Phil Lester.
The "Dan and Phil LICK RACE" was published on the AmazingPhil channel, which has more than 2.8 million subscribers, as part of a campaign run by Mondelez, which owns Oreo. YouTubers Thomas Ridgewell, Emma Blackery, from Essex, and PJ Ligouri also posted similar videos.
The vloggers were criticised in the ASA ruling for repeatedly referring to the biscuit brand without making clear the videos were adverts.
The comprehensive new guidance, issued by the Committee of Advertising Practice (CAP), an ASA body, calls on vloggers to make it immediately clear if they were paid to promote a product or service.
CAP also said it would work with advertisers to ensure vloggers are not pressured into featuring products without making it clear they are adverts, following complaints from some YouTubers.
But vloggers and celebrities will still be able to feature free gifts in videos posted online, as long as their reviews are not influenced by the brand.
The soaring popularity of online video blogs has created a lucrative industry, with teen sensation Zoe Sugg, better known by her YouTube name Zoella, taking home estimated earnings of £300,000 last year from her lifestyle blog.
But the trend has fuelled an increase in product placement, which is seen as deceptive by some fans, who believe stars are endorsing products they genuinely like.
Kim Kardashian was last week forced to delete a picture from her Instagram account, which US regulators said promoted a morning sickness drug, but did not mention the health risks associated with taking it.
In 2012, footballer Wayne Rooney was also found to have breached advertising rules, promoting sports brand Nike on his Twitter account without making clear it was a paid advert.
Director of CAP, Shahriar Coupal, said the rules would give vloggers "greater confidence" and "in turn help maintain the relationship and trust they've built with their followers".
He said: "Wherever ads appear we should be confident we can trust what an advertiser says; it's simply not fair if we're being advertised to and are not made aware of that fact."