Britain's advertising watchdog on Wednesday banned two adverts by clothing brand American Apparel for being offensive and normalising sexually predatory behaviour.

The adverts showed a model wearing a pleated school-style skirt bending over, with a low camera angle revealing her underwear.

Two people had complained about the adverts, saying as they were inappropriate for a skirt advertised as school wear. The adverts also attracted criticism on social media when they first appeared.

"We considered the images were gratuitous and objectified women, and were therefore sexist and likely to cause serious and widespread offence," the Advertising Standards Authority (ASA) said.

The watchdog said the images "imitated voyeuristic 'up-skirt' shots which had been taken without the subject's consent" and therefore had the potential to normalise "predatory sexual behaviour".

"From the context in which the ads appeared, it was likely that those who viewed them would understand that the model was, or was intended to appear to be, a schoolgirl," the watchdog said.

"We considered the ads had the effect of inappropriately sexualising school-age girls and were therefore offensive and irresponsible."

American Apparel defended the adverts, saying the model was a 30-year old woman, that the ads were not pornographic, and that they had appeared only briefly on the company's website and Instagram page.

It also said the images were not part of a back to school campaign, but this was disputed by the ASA which said the ads had featured in its "school days" collection of photographs.

Known for its racy advertising, American Apparel ousted its founder and CEO Dov Charney in June following an investigation into alleged misconduct and a series of sexual harassment allegations.