The ad, which screened on ITVBe during the programme Dinner Date in April, featured two young females modelling various outfits in the middle of a desert plain.
A viewer complained to the Advertising Standards Authority (ASA) that the ad was irresponsible for portraying models who appeared to be children in a sexualised manner.
21 Three Clothing, trading as Pretty Little Thing, said the models were 23 and 24 years old, and the styles of clothing worn in the ad were on-trend pieces designed for the festival season.
Ad clearance agency Clearcast said its viewing panel felt that the tone of the ad was in line with the summer or festival spirit of the products and the models featured were neither childlike nor indecent or overly sexualised.
The ASA noted that one model had a "very youthful appearance", some of the outfits were tight fitting or revealing and some poses drew attention to the models' chests and bottoms.
The watchdog said the combination of the models' poses, their expressions and their outfits meant that the ad was sexually suggestive.
It said: "We considered that the ad portrayed a model who appeared to be under 16 in a manner that was sexually suggestive.
"We therefore concluded that the ad was irresponsible and in breach of the Code."
It ruled the ad should not appear again, adding: "We told Prettylittlething.com to ensure that future ads did not sexualise those who appeared to be children."