France has sent shock waves through the global fashion industry by passing a surprise law making it a criminal offence to employ dangerously skinny women on the catwalk.

Under the new law, anyone running an agency found employing undernourished models below an as-yet undefined Body Mass Index, or BMI, risks a maximum six-month prison term and a €75,000 (£55,000) fine.

Magazines will also have to systematically indicate when a photograph of a model has been digitally "touched up" to make her look skinnier or bulkier on pain of a €37,500 fine or up to 30 per cent of the sums spent on advertising.

A model presents a creation by French designer Lea Peckre as part of her Spring/Summer 2015 women's ready-to-wear collection during Paris Fashion Week September 23, 2014 (Reuters / Gonzalo Fuentes)

The French fashion world reacted with anger, however, and described the measure as "a dangerous confusion between anorexia and the slimness of models" that will disadvantage Gallic agencies in what is a global industry.

French "fashionistas" were already up in arms against another amendment passed on Thursday night that will make glorifying anorexia on the internet a criminal offence. People who run so-called "pro-ana" or "thinspirational" websites risk a maximum year's imprisonment and a fine of €10,000 for "provoking people to excessive thinness by encouraging prolonged dietary restrictions that could expose them to a danger of death or directly impair their health". A host of website or blogs claim to offer beauty tips to girls as young as 12, including starving themselves to create stick legs and a yawning "thigh gap".

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Until Friday morning, it was assumed the second measure targeting modelling agencies would be dropped, as many MPs had pointed out that it would violate France's strict employment law on discrimination in job recruitment.

However, the National Assembly voted it through after health minister Marisol Touraine said the use of excessively thin models in fashion was "worrying" and that she backed the change.

A file photo taken on March 10, 2015 shows a model presenting a creation by Iris Van Herpen during the 2015-2016 fall/winter ready-to-wear collection fashion show in Paris (AFP / Getty)

Dr Olivier Véran, a Socialist MP and neurologist who tabled the amendment, said it was crucial to change mentalities in the fashion world about what is considered acceptable in terms of skinniness.

"The prospect of punishment will have a regulatory effect on the entire sector," he said, pointing out that Spain, Italy and Israel had already taken similar measures.

Spain bars models below a certain body mass index from featuring in the Madrid fashion shows; Italy insists on health certificates for models and Brazil is considering demands to ban under age, underweight models from its catwalks.

The World Health Organization considers people with a BMI below 18.5 to be underweight and at risk of being malnourished.

Arnaud Robinet with the opposition centre-Right UMP party said the new law was "inapplicable and discriminatory" and would put the French fashion world at a disadvantage.

"Agencies will employ foreign models over French models," he claimed.

France's national model agency union Synam said it a purely French approach would disadvantage the country's models.

"French model agencies are constantly in competition with their European counterparts. As a result, a European approach is essential," it said.

It also said the law "confuses anorexia with the slimness of models".

"When you look at criteria for anorexia, you can't just take BMI into consideration, but other criteria too, psychological but also whether models are losing hair or have teeth problems (due to undernourishment)."

The new law will ban any model whose BMI falls below a level fixed by France's Higher Health Authority.

But Isabelle Saint-Felix, secretary general of the union, insisted that some models, such as Ines de la Fressange, often dubbed the quintessential twiglike French model, were skinny by nature.

"She says herself that its part of her constitutional make-up, just like the rest of her family".

Dr Véran said a 2008 charter of good practice signed by the fashion industry had failed to change mentalities. During the parliamentary debate, he read out a letter from a top model explaining how agencies "congratulated girls who lose weight and recommend taking laxatives".

He cited one model who weighed "less than 45kg at 1.8m tall" whose friend died of a heart attack after coming off the catwalk due to "starvation".

There are an estimated 40,000 people in France suffering from anorexia, around 90 per cent of whom are adolescents.