Madrid's fashion week is banning underweight models in response to fears that their skinny frames are responsible for a rise in the number of eating disorders among young women who try to copy the way they look.

Organisers say the industry has a responsibility to portray healthy body images rather than what's being dubbed as "heroin chic."

Models are instead being hired on the basis of their body mass index - a measure of their weight in relation to their height. Under the Madrid ruling models must have a BMI rating of around 18, and doctors will be on hand at fashion week, which begins on 18 September, to check the models.

Around a third of the women who took part in last year's show have failed to meet the new restrictions according to the Association of Fashion Designers of Spain.

The decision to portray healthy body images was made as part of a voluntary agreement with the Madrid regional government, which sponsors the show, also known as the Pasarela Cibeles.

"The restrictions could be quite a shock to the fashion world at the beginning, but I'm sure it's important as far as health is concerned," Leonor Perez Pita, director of Madrid's show, was quoted as saying by Reuters news agency.

This is believed to be the first ban on the use of extremely thin models, and has outraged some sections of the fashion world. Modelling agencies say the fashion industry is being used as a scapegoat for weight-related illnesses, and that some models' careers could be damaged.

Spain's actions have also raised the prospect of restrictions at other catwalk pageants.

The mayor of Milan, Italy, Letizia Moratti told an Italian newspaper she would seek a similar ban for her city's show unless it could find a solution to "sick" looking models, although London Fashion Week said it would not try to restrict the type of models its designers use.