Video bloggers - or "vloggers" - need to make it clear when they are paid to promote products, the Advertising Standards Authority (ASA) has warned.
The new guidance to vloggers was issued following an investigation carried out by BBC Newsround.
Several UK vloggers were paid to praise Oreo biscuits, but none of the videos was clearly labelled as an advert.
Mondelez, the biscuit company behind the adverts, said it was disappointed but would not show the videos again.
The ASA ruled that the promotions were not clearly marked, and must not appear again in their current form.
Lynsay Taffe from the ASA stressed that brands and vloggers would have to make it clear before the video is viewed that it is promotional.
That means if a vlogger is paid to promote a product, they need to put something like the word "ad" or "promo" in the title of their video - or use a symbol in the thumbnail telling viewers what they're about to click on is an advert, she explained.
'Clear and upfront'
She added: "Vloggers often have huge followings built on authenticity, built on them providing interesting, funny, natural content.
"We think it's only fair that when they start promoting stuff on behalf of a brand - which is absolutely fine for them to do - that they do so in a way that's clear and upfront with their audience."
Labour MP and former Culture Secretary Ben Bradshaw told Newsround he was worried that this case was part of a wider problem.
"There are strict rules that govern television and other advertising and it seems to me that there's a bit of a loophole when it comes to online, videos and YouTube," he said.
Chyaz - a lifestyle and beauty vlogger with more than 45,000 subscribers to her YouTube channel - said companies sponsoring her videos had told her: "Please don't make it very obvious."
She was not one of those paid to promote Oreos but said: "It hasn't been clear at all.
"Everybody's tried to do what they think is right but it's nice to have something more solid - some guidelines so we can do what's actually right."
Another vlogger, Mawaan Rizwan, told the BBC he had been happy to take money from advertisers in the past, but now takes more cautious approach.
"YouTube is essentially a democratic platform," he said. "With big corporations coming in and going: 'Hey, can you hold this sugary drink next to your face in your video,' it's killing the community aspect of YouTube."
He argued: "It's important for advertisers to have respect for YouTubers' style of content and not just crowbar in their brand when it doesn't fit."
Mondelez, the firm that paid the vloggers to promote its biscuits, said it would "ensure the adverts did not appear in their current form again".
Social-networking and video site YouTube told Newsround that the vloggers themselves were responsible for making videos that stick to their local laws and regulations.
The vloggers affected by the ASA ruling have now changed the descriptions of their videos to make it clear they are paid advertisements, but the ASA said it would be looking at online videos much more closely in future.