The image is one of a carousel of pictures on the American Apparel website showing a young model wearing a spandex bodysuit.
Since the bodysuit has a thong, one shot shows the model from behind with a graphic depiction of her buttocks.
The photo was banned by the UK’s ad watchdog after a complainant pointed out that the model looked under 16. The Advertising Standards Authority ruled the image "could be seen to sexualise a child", breaching rules on social responsibility and causing offence.
It concluded the "inappropriate" image could cause "serious offence".
In its defence, American Apparel said the model was 20 years old and that the image was consistent with similar ads.
The ad must not appear in its current form again in the UK, and the ASA warned the brand against images that sexualise young women. This is the second time in six months the brand has had an ad banned by the ASA, after two images from its ‘back to school’ campaign were blocked in September.