Celebrities like Jennifer Lopez, Diddy, and Ciara were warned by the FTC about their sponsored Instagram posts

Larry Busacca/GettyCelebrities including Jennifer Lopez, Sean "Diddy" Combs, and Naomi Campbell were among those who received warning letters from the FTC in April over sponsored Instagram posts, Women's Wear Daily reports.

The 90 letters - which were obtained by WWD's Alexandra Steigrad - were sent to 45 celebrities and the brands they were promoting. The warnings marked the first time the FTC had reached out directly to social media influencers.

The letters were intended to clarify the type of language used to state the post was sponsored (like #ad) and where in the caption it should say that.

"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 FTC said when the letters were initially sent in April.

Here are some of the celebrities and brands that got into hot water. For the full list, head over to WWD.