The Competition and Markets Authority (CMA) is concerned media influencers, who have millions of followers watching their channels, may be falling foul of consumer protection laws that require posts to be properly labelled.
It is also worried followers may place trust in a product endorsed by a star and may think it is their own personal view.
Online endorsements from celebrities and influencers can help brands reach target audiences and boost sales.
Media influencers like Kylie Jenner, Selena Gomez, and footballer Cristiano Ronaldo can earn hundreds of thousands of pounds for an endorsement.
Jenner commands $1m (£760k) per Instagram post, according to UK social media marketing firm Hopper HQ.
George Lusty, the CMA’s senior director for consumer protections, said: “Social media stars can have a big influence on what their followers do and buy.
“If people see clothes, cosmetics, a car, or a holiday being plugged by someone they admire, they might be swayed into buying it.
“So, it’s really important they are clearly told whether a celebrity is promoting a product because they have bought it themselves, or because they have been paid or thanked in some way by the brand.”
The CMA said it has seen examples of posts which appear to promote or endorse products without clearly stating if the post has been paid for.
It has also seen posts where celebrity’s offer their personal opinion on the benefit of a product without clearly disclosing if they are being paid by the brand.
As part of its investigation, the CMA has written to celebrities and social media influencers to gather more information about their posts and the nature of the business agreements they have in place with brands.