The Federal Trade Commission (FTC) has warned a number of "influencers" – social media celebrities with large numbers of followers – that they must disclose any financial arrangements with brands mentioned in Instagram posts.

The Wall Street Journal reported Friday that the trade regulator sent letters to 21 Instagram personalities in September, telling them to disclose arrangements with any companies or brands mentioned in recent posts on the photo-sharing app.

Model Naomi Campbell and actresses Shay Mitchell and Lucy Hale were among those to receive the warning letters from the FTC, the Journal reported.

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“There are tons and tons of endorsements out there,” Michael Ostheimer, an FTC attorney, told the Journal. “Some people are making adequate disclosures. Some are not.”

Brands have courted influencers in recent years as a way to reach potential customers on social media. But according to the Journal, regulators are looking at such influencers for potential violations of rules against deceptive marketing practices – even those given financial rewards from brands without promising anything in return.

According to the Journal, several such influencers said that they did not know they could be violating deceptive marketing rules.