Truth in Advertising, a not-for-profit organisation, has found more than 100 Instagram posts that break US law because they fail to show they are paid-for ads.
The organisation has sent a letter to the family and the companies it works with to change the illegal posts within a week or it will report the group to the Federal Trade Commission.
Anyone posting an ad on their social media account must make it clear. For example on Twitter, they can use #ad if there is limited space.
Bonnie Patten, executive director at TINA.org, said: "When it comes to sponsored social media posts, the law is clear – unless it’s self-evident that an Instagram post is an advertisement, a clear and prominent disclosure is required so that consumers understand that what they are viewing is an ad.
"The Kardashian/Jenner family and the companies that have a commercial relationship with them have ignored this law for far too long, and it’s time that they were held accountable."
Earlier this year there were new international guidelines issued which said that all online influencers must disclose paid-for sponsored posts in any content.