The US Federal Trade Commission (FTC) has sent out letters to 21 people with large Instagram followings asking whether they have been paid to promote certain products on the platform.

The list of Instagram celebrity users to receive the letters, which were obtained by Reuters, includes Naomi Campbell, Lindsay Lohan, Vanessa Hudgens, Snooki, Amber Rose and Scott Disick. They have been contacted as the FTC seeks to get a handle on the rise of influencer posts that are missing any form of disclosure that they are adverts.

The letters were sent out on 6 September, and the FTC demands that the celebrities respond by the end of the month to its concerns around certain posts.

In the case of Naomi Campbell, the FTC points to a post that features three suitcases made by Globe-Trotter with no disclosure attached. If there was indeed a marketing deal in place for the post, then the FTC expects Campbell to tell it what actions she will be taking to ensure disclosure in the future.

Earlier this year, Instagram made efforts to make it easier for influencers to disclose when their posts have been created for commercial purposes. It introduced a ‘paid partnership with’ tag for creators and businesses to use on any of their branded content. This followed the FTC issuing a warning to influencers in April about the need for disclosure.