An Instagram post by reality star Jemma Lucy has been banned by the Advertising Standards Authority (ASA).
The ASA said the image and text broke several rules, including encouraging unsafe practices during pregnancy and making claims about weight loss.
The post was also not properly identified as an advert, it said.
The ASA recently teamed up with ITV to create a checklist to encourage Love Island contestants to be "upfront" about their own social media ads.
Skinny Caffe, the weight-loss brand promoted by Lucy, said she had posted the message as a favour to a friend and had not been paid for it.
The firm was seeking to create brand awareness ahead of the model giving birth, and had planned a larger follow-up campaign later in the year.
Skinny Caffe said it did not believe Lucy had implied that she had used its products while pregnant.
Lucy concurred with Skinny Caffe's comments but acknowledged the text contained in her message had been provided by the brand.
The ASA said it had investigated the matter after receiving 25 complaints from the public.
Lucy rose to fame via a succession of reality TV series, including Signed by Katie Price, Ex on the Beach, and Celebrity Big Brother.
She uploaded the offending post on 5 May.
It showed the glamour model sitting by a table wearing underwear and socks, cradling a mug next to the products.
The ASA noted that although this post did not refer to her pregnancy, comments under many of her other Instagram posts did. Moreover, it said that her pregnancy had been widely covered in the press.
In light of this, the regulator said her fans might take the text's claim that she had used the products to stay "in shape" to imply she had consumed them recently. And, it added, this could encourage other mothers-to-be to do likewise.
The ASA noted that the National Institute for Health and Care Excellence does not recommend dieting during pregnancy as it can harm the health of the unborn child.
"We concluded that the ad encouraged an unsafe practice and was irresponsible," the watchdog concluded.
It also highlighted the fact that the text also claimed the products featured could help users "lose up to 7lbs [3.2kg] in seven days".
According to the regulator, any health claims that refer to specific amounts of weight loss are not permitted in ads.
The ASA does not have the power to issue fines, but can have problem ads taken down and works with platforms to have them removed.
In serious instances, the authority can also refer incidents to Trading Standards, which does have the power to impose fines and prosecute offenders.
The BBC has contacted Jemma Lucy for comment.