The Advertising Standards Authority (ASA) upheld a complaint that the ads were "exploitative" and likely to cause "serious and widespread offence".
The ads, placed on American Apparel's website in October 2011 and in a free lifestyle magazine, featured young women wearing the firm's clothing in sexually suggestive poses, often with their buttocks and breasts exposed.
A complainant contacted the ASA to express concern that the ads were "offensive", "pornographic", and "exploitative of women", as well as claiming they "inappropriately sexualised young women".
American Apparel denied all the claims. The retail chain said that the images featured "real, non-airbrushed, everyday people", rather than professional models.
It is also asserted that these sorts of images were regularly shared by people with their friends on social networks, meaning consumers could relate to them.
In its ruling, the ASA noted that some of the adverts were for lingerie items, meaning it was "reasonable" to feature women with limited amounts of clothing.
However, the regulator said that the majority were for outer garments, and so there was no justification for buttocks and breasts to be exposed.
"[We] considered that the nature of the women's poses meant that their breasts and buttocks were the focal points of the images rather than the products. We considered that the nudity was therefore gratuitous," said the ASA.
The regulator also said that the poses of the women in the ads, emphasising their breasts and hips, were sexually provocative. This, combined with the gratuitous nature of the images, meant they were also "exploitative and inappropriately sexualised young women".
American Apparel was told by the ASA that the ads must not appear again in their current form, and warned not to use "similar images which were exploitative of women or that inappropriately sexualised young women" in the future.
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