Celebrities including Ellie Goulding, Rita Ora and Michelle Keegan have agreed to change the way they label their social media posts.
A total of 16 stars will now clearly say whether they have been paid in exchange for a post, or if an item they are featuring has been gifted or loaned to them.
Others agreeing to the pact include models Alexa Chung and Rosie Huntington-Whiteley - as well as reality TV stars Mario Falcone and Millie Mackintosh.
Falcone's Twitter account heavily features a luxury watch brand, with tweets showing him embracing his newborn son while endorsing a 10% discount code. Meanwhile, Mackintosh's Instagram is packed with mentions of beauty and clothing brands.
Together, the celebrities have tens of millions of followers across platforms including Instagram and Twitter.
The formal commitment follows action by the Competition and Markets Authority (CMA), which wants to improve transparency and prevent consumers from being misled by celebrity endorsements.
Such posts can have a powerful impact and boost sales for the companies promoted - with millions of fans exposed to updates about where the celebrities go on holiday, what they wear, and the products they use.
By law, stars are required to disclose whether they have been paid to incentivise or endorse a brand.
Several celebrities have been sent warning letters urging them to review their practices.
Andrea Coscelli, the chief executive of the CMA, said the move sends a "clear message" to influencers, brands and businesses about what is expected from them.
He added: "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 CMA has also published a set of guidelines for social media influencers, called An Influencer's Guide To Making Clear That Ads Are Ads.
It has not come to any conclusions about whether posts have broken the law.
© Sky News 2019