The ad, which appeared in Elle U.K. magazine, showed a woman in a short black dress and high heels sprawled on the floor with her eyes closed.
The Advertising Standards Authority says the model's pose drew attention to her visible rib cage and to her legs, "where her thighs and knees appeared a similar width."
It ruled Wednesday that the model appeared unhealthily underweight and that the ad was irresponsible. It said the ad must not appear again.
The watchdog said the fashion house did not agree with its view on the model's weight, but did not provide a detailed response.