As Madrid's fashion week bans models with a Body Mass Index below 18 and Milan threatens to follow suit, Maxine Frith weighs up whether the catwalks are finally waking up to their responsibility to promote healthy eating

The world's most fashionable circus sweeps into London next week, when the capital hosts the latest round of international designer catwalk shows.

Julien Macdonald, Jasper Conran, Ben De Lisi and Zandra Rhodes are among the top names exhibiting their Spring/Summer collections for 2007 in the imposing environs of the Natural History Museum.

This season's must-have trends, as evidenced in New York this week, include outsized suits and billowing dresses.

But the question of size that is on every fashionista's lips just now is not about the width of the trouser legs - but the girth of the models slinking down the catwalk.

British eating disorder experts yesterday called for a new law under health and safety legislation that would ban the use of models under a certain body size. They said that after years of promising to clean up its act voluntarily, the fashion industry had to be forced into protecting both the health of the models it uses, and the impact their images can have on impressionable young girls and women.

Steve Bloomfield, of the Eating Disorders Association, said: "We do think legislation is needed. The industry will not act voluntarily because the fashion world is so competitive and no-one wants to be the first to do anything in case they lose out.

"A law would create a level playing field. Agencies would not be able to hire models below a certain body mass index (BMI) and magazines and designers would not be able to use images of them.

"This is about protecting the young women and men who work in the fashion industry, as well as those who are at risk of an eating disorder, and can be influenced by the pictures that they see."

The tide finally appears to be turning against the fashion industry's use of super-thin models to promote its wares. Last week, Madrid City Council, which is the sponsor and regulator of the Spanish capital's annual fashion week, announced that it was imposing a ban on the use of models with a BMI of less than 18.

BMI is seen as a reliable way of calculating whether someone is a healthy size. The statistic is found by dividing weight in kilograms by square of height in metres.

The Spanish rules would mean that a model who is 5ft 9in tall will have to weigh at least 8st 11lb to work.

According to estimates, the average catwalk model is 5ft 9in tall and weighs just 7st 12lbs - giving a BMI of only 16.

Mr Bloomfield said a cut-off point of a BMI of 18 was appropriate because at that level, an adult was considered to be at significant risk of an eating disorder, while a rating below that meant it was "highly likely" that a disorder already exists.

Among those who would allegedly be refused access to the Madrid catwalks would be the Spaniard Esther Canadas and British star Kate Moss, who, according to insiders, has a BMI of around 15 although even she is now not among the skinniest models in the industry.

The world's fashion capital, Milan, may also follow the trend set by Madrid.

The city's mayor, Letizia Moratti, yesterday warned that she will seek a similar ban at the city's fashion week - one of the most influential in the world - unless the fashion industry changed its stance on using "sick-looking" models.

"If the industry cannot find a mature solution to the problem, we will impose one," she said. But it remains to be seen whether the rest of the notoriously fickle fashion world follows suit - and the signs are not good.

Riccardo Gay, head of the Milan-based modelling agency of the same name, insisted that he had never taken on an anorexic model and that all his models were healthy.

Beppe Modenese, honorary president of the National Chamber of Fashion, insisted: "Now the women on the catwalk are normal girls, the age of Twiggy is over."

The British Fashion Council (BFC), which organises London's week of shows, has said that it has no plans to introduce a BMI limit on its designers.

Hilary Riva, chief executive of the BFC, said: "We have canvassed the opinions of London Fashion Week designers for the girls they would most like to see on their catwalks.

"The BFC does not comment or interfere in the aesthetic of any designer's show."

Sarah Doukas, Kate Moss's agent at Storm Models, dismissed the BMI regulation idea during an interview last week.

She said: "It's useless to talk about BMIs.

"I believe that girls should just eat healthily, exercise and just be normal.

"We just wouldn't use someone who was really underweight or too thin."

Most of the top London Fashion Week designers contacted by The Independent, including Ben de Lisi, Jasper Conran and Paul Smith, failed to respond to requests for a comment about the proposed legislation.

Only the Gharani Strok label issued a comment, saying: "We would never use a girl that we felt looked ill or had an eating disorder as we love our girls beautiful and healthy."

However, industry insiders have said privately that they would welcome legislation rather than a voluntary code of conduct, because it would ensure that no-one was able to steal a march on the competition.

Government attempts to broker a voluntary agreement on the use of thin models in fashion magazines six years ago floundered when the most influential editors refused to back the idea.

Tessa Jowell, the then Minister for Women, hosted a "body image summit" along with Liz Jones, then editor of Marie Claire magazine.

They announced proposals for a voluntary code of self-regulation that would try to ensure that models of more varied (for that, read bigger) sizes were used by the fashion industry.

But in a graphic example of just how vicious the industry can be, the editors of rival magazines issued an open letter the very next day saying they had no absolutely intention of signing up to such a scheme.

Ms Jones, who had tried to introduce bigger-sized models to the pages of Marie Claire and even asked readers to pick between competing covers of Sophie Dahl and Pamela Anderson, was later sacked as editor, while Ms Jowell's women's unit was quietly abandoned a year later.

The magazine industry, designers and model agencies claim that they only supply what the public or their clients demand, but Dr Dee Dawson, who runs the Rhodes Farm eating disorders clinic in north London, says she has little truck for these arguments.

She said: "They have had years to get their house in order and they have done absolutely nothing - in fact if anything it has got worse.

"I tell the girls at my clinic about how in my day, it was all about Twiggy - she was so unusual and bizarre looking because she was so skinny - yet now that kind of size is the norm for catwalks.

"My girls have Victoria Beckham as a role model - a woman who is so skinny that you can see every rib on her and the only reason she looks even vaguely normal is because she has had a boob job.

"The fashion industry has completely warped what is considered a normal size, and it should be held accountable for that."