Sixteen social media celebrities, including pop stars Ellie Goulding and Rita Ora and models Alexa Chung and Rosie Huntington-Whiteley, have agreed to change the way they post online.
They will have to clearly state if they have been paid or received any gifts or loans of products they endorse.
It follows warnings from the Competition and Markets Authority that such posts could break consumer law.
Online endorsements can boost brands but can also mislead, said the CMA.
Andrea Coscelli, chief executive of the CMA, said: “Influencers can have a huge impact on what their fans decide to buy. People could, quite rightly, feel misled if what they thought was a recommendation from someone they admired turns out to be a marketing ploy.
“You should be able to tell as soon as you look at a post if there is some form of payment or reward involved, so you can decide whether something is really worth spending your hard-earned money on.”
The practice of endorsing products, from clothes to cars, hotels to holidays, is widespread among so-called social media influencers who can earn tens of thousands of pounds from companies for a single post on sites such as Instagram.
The risk is that consumers will place more trust in a product that has been recommended by someone they admire on social media.
Some celebrities do use hashtags such as #ad or #sponsored to indicate that they have been paid to post content.
The CMA has not made a finding on whether the influencers in question have breached consumer law, but instead stressed that all have volunteered to change their practices.
It hopes their examples will serve as warnings to others.
If the celebrities fail to comply with the agreement reached with the CMA, they could be taken to court and face heavy fines or prison sentences of up to two years.
Others who have agreed to change the way they post on social media are:
- Zoe Sugg (who vlogs under the name Zoella)
- Michelle Keegan
- Mario Falcone
- Alexandra Felstead
- Holly Hagan
- Iskra Lawrence
- Camilla Macintosh
- Megan McKenna
- Chloe Sims
- Louise Thompson
- James Chapman (who vlogs as Jim Chapman)
- Dina Torkia
Warning letters have been sent to a number of other unidentified celebrities, urging them to review their social media posts.
The CMA will be conducting further investigations into the role and responsibilities of social media platforms.
The Advertising Standards Authority launched its own investigation into advertising on social media following complaints about the practice and has flagged up content from reality TV star Louise Thompson about product endorsement on two occasions.