Madrid fashion week, one of Spain's most prestigious shows, is banning underweight models on the basis of their body mass index (BMI).
The Madrid fashion week runs from 18 September
UN health experts recommend a BMI of between 18.5 and about 25, and some models may fall well below the minimum.
The Spanish Association of Fashion Designers has decided to ban models who have a BMI of less than 18.
Unhealthily skinny models at last year's fashion shows led to protests from doctors and women's rights groups.
The association agreed to use the BMI - a calculation based on height and weight - in response to local government pressure.
It suggests that 30% of would-be participants fail this test and this year's fashion week, which begins on 18 September, will offer medical treatment to excessively thin models.
"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 the Madrid fashion show, was quoted as saying by Reuters news agency.
Madrid's local government says it wants to set a more positive, healthy image of beauty for teenagers to follow.
"Fashion is a mirror and many teenagers imitate what they see on the catwalk," said regional official Concha Guerra.
Spain's Anorexia and Bulimia Association says if designers refuse to follow these voluntary restrictions the government should legislate to ban thin models.
However, some sections of the fashion world have expressed outrage at the idea of weight restrictions.
Cathy Gould, of New York's Elite modelling agency, said the fashion industry was being used as a scapegoat for weight-related illnesses.
"I understand they want to set this tone of healthy beautiful women but what about discrimination against the model and what about the freedom of the designer?" she asked, adding that the careers of naturally "gazelle-like" models could be damaged.