Celebrities and "influencers" on Instagram have been warned they risk breaching consumer rights rules by not revealing when they are promoting products on the social media site.
The US consumer regulator has sent a letter to 90 high profile users of the Facebook-owned site in response to concerns raised by consumer rights group Public Citizen over paid endorsements on some Instagram posts.
Public Citizen said many endorsement posts were not being clearly labelled as such, breaching the Federal Trade Commission's (FTC) endorsement guidelines, and citing 113 posts by stars including Rihanna, Rita Ora and Kylie Jenner as examples.
No accounts or users were named by the FTC in their warning.
The Commission said its warning letters told influencers they must "clearly and conspicuously" disclose when they have been paid to endorse a product or received it for free in return for promotion, and offered guidance on how to present these posts.
In a statement the Commission said: "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.
"The staff's letters informed recipients that when making endorsements on Instagram, they should disclose any material connection above the 'more' button."
The Commission also warned that including endorsement references within a string of hashtags could still be considered a breach of the guidelines.
"The letters also noted that when multiple tags, hashtags, or links are used, readers may just skip over them, especially when they appear at the end of a long post - meaning that a disclosure placed in such a string is not likely to be conspicuous," they said.
The FTC also said use of the hashtags '#sp' - short for sponsored - or '#partner' were insufficient ways of alerting users to endorsement posts.
This is the first time the Commission has intervened on the issue on social media, but last year did reach a settlement with Warner Bros over claims the firm did not disclose it had paid high-profile YouTubers to promote one of its video games.