In the advert, Alabama Barker is seen clad in an all-pink outfit with a pink mini dress, pink high heels and orange sunglasses.
She is also holding a hosepipe which is spraying a nearby hedge. The overlaying text of the advert read: “Channel that teen dream realness with barely-there micro mini skirts.”
The ASA said the image “revealed her breasts” and another image from the campaign saw her wearing a “tight-fitting short dress whilst sucking a lollipop”.
The ASA said the advert was banned as it was “in breach of our rules which state that ads must not portray or represent anyone who is, or seems to be, under 18 in a sexual way”.
It added that it deemed that advert to be “socially irresponsible” and “likely to cause harm and offence”.
In response to the ban, PrettyLittleThing confirmed that Barker was 16 years old at the time of the shoot and said they had chosen Barker as a brand ambassador because their customer base was primarily aged between 16 and 24-years-old.
PrettyLittleThing added that they “did not intend” to sexualise Barker and disagreed that she was portrayed in a sexual manner.
The retailer added that the images were approved by Barker and her team, and that Barker sucking a lollipop was a nod to the Y2K trend the advert was portraying.
As for the image with the hose, PrettyLittleThing said they “believed cooling down on the lawn on a hot day fitted the intended Y2K aesthetic”.
Barker clutched her chest in some of the images for the purpose of the photoshoot, it continued, and that use of open mouth poses was popular among young people on social media.
PrettyLittleThing added they wanted to convey a message of body positivity to “encourage and empower young women to embrace their bodies and inspire confidence”.
The Independent has contacted PrettyLittleThing for further comment.