An Elle UK reader has complained that the YSL advert was 'irresponsible'
An Yves Saint Laurent advertisement featuring an "unhealthily underweight" model has been banned by the Advertising Standards Authority (ASA).
The advert, which appeared in Elle UK magazine, featured a photo of a woman whose rib cage was visible and appeared prominent, the regulator said.
It upheld a reader's complaint that the advert was "irresponsible" for using a model who appeared unhealthily thin.
Yves Saint Laurent (YSL) and Elle UK declined to comment on the ruling.
The ASA said YSL "indicated that they did not agree with the complainant's view that the model was unhealthily thin" but did not provide a detailed response.
The ASA said the advert drew attention to the model's visible rib cage, and knees and thighs of a similar width
Elle UK told the watchdog it had no comment to make on the complaint.
The ASA said the model's pose and the lighting drew particular focus to her chest, where her rib cage was visible and appeared prominent, and to her legs, where her thighs and knees appeared a similar width.
It said: "We therefore considered that the model appeared unhealthily underweight in the image and concluded that the ad was irresponsible."
It ruled that the advert must not appear again in its current form, adding: "We told the advertisers to ensure that the images in their ads were prepared responsibly."
The charity Anorexia and Bulimia Care (ABC) said it hoped the ASA's ruling sends a "clear message" to other media and fashion companies, which it said have a "great responsibility".
An ABC spokesperson told the BBC: "We applaud the ASA for taking the necessary action to ban the YSL advert on the grounds of it being 'irresponsible.'
"While eating disorders are most often caused by underlying emotional issues or events, the impact of the media on vulnerable young people can act as a dangerous catalyst - triggering disordered thinking and behaviour.
"Adverts using underweight models are promoting a distorted image of beauty and yet this has become the norm in the UK."
The ASA's ruling came as figures from the NHS showed that the number of hospital admissions across the UK for teenagers with eating disorders has nearly doubled in the last three years.
The Royal College of Psychiatrists said much of the increase was down to social pressure made worse by online images.
College spokesperson Dr Carolyn Nahman said she was worried about what she described as the sometime fatal consequences of vulnerable teenagers putting themselves under pressure by looking at pictures of "ideal bodies" repeatedly on social media.
Earlier this year, a campaign for the YSL perfume Black Opium was cleared by the ASA following complaints that it glamorised and trivialised drug use and addiction.