It comes after adverts by other retailers were banned earlier this year for models being too thin.
In July, an advert by Marks & Spencer was banned because the model appeared to be “unhealthily thin”.
The ASA said the pose of the model and the choice of clothing – including “large pointed shoes” which emphasised “the slenderness of her legs” – made the advert “irresponsible”.
Earlier this year, fellow retailer Next also had an advert for blue skinny jeans banned.
The ASA said the advert emphasised the thinness of the model’s legs using camera angles, and deemed it “irresponsible”.
Next said it disagreed with the advertising watchdog’s decision and said the model, while slim, had a “healthy and toned physique”.
The Next advert ban left BBC readers asking why adverts showing models who appear unhealthily overweight are not banned.