90 celebrity influencers have received warnings regarding paid advertisements on Instagram and other social media sites.

The US consumer regulator, Federal Trade Commission (FTC), has sent letters to 90 popular social media users warning them about the dangers of promoting products on Instagram without making it clear to their followers that it is a paid advertisement. The social media promotion raised red flags according to consumer rights group, Public Citizen, who claimed these types of posts were in breach of the FTC’s endorsement guidelines. The group used posts from stars including Rihanna, Kylie Jenner and LeBron James as examples of the breach.

How cool is this party we are hosting with @bumble? It's snowing in the desert! Having the best time here at the #WinterBumbleland party! #ad ❄??

A post shared by Kylie (@kyliejenner) on Apr 15, 2017 at 4:43pm PDT

The FTC said that their warnings stated that advertisements must be “clearly and conspicuosly” tagged as a paid promotion, and followers must be made aware when an influencer has received a product for free. “In addition to providing background information on when and how marketers and influencers should disclose a material connection in an advertisement, the letters each addressed one point specific to Instagram posts – consumers viewing Instagram posts on mobile devices typically see only the first three lines of a longer post unless they click ‘more’, which many may not do,” read the Commission’s statement on the issue. “The staff’s letters informed recipients that when making endorsements on Instagram, they should disclose any material connection above the ‘more’ button.”

Detoxing with @fittea ? it tastes amazing and the ingredients are all natural ? #ad

A post shared by Kim Kardashian West (@kimkardashian) on Mar 27, 2017 at 8:22am PDT

The news will come as a blow to stars like Kim Kardashian, Gigi Hadid and Michael Phelps who can all earn $100,000 to $250,000 for a single endorsed post, according to experts.