Fashion labels Prada, Versace and Armani have agreed to ban stick-thin women from their catwalks, prompted by the death of two models.
The Italian couture houses have united with their government to agree new rules due to come into force before Milan's fashion week in February.
Milan stands alongside Paris and New York as one of the world's most important fashion events. Like the other two it has, until now, resisted demands to stamp out the waif look.
But Camera Nazionale della Moda Italiana, which represents the country's top labels, changed stance after Brazilian model Ana Carolina Reston died last month of an illness caused by anorexia. Uruguayan model Luisel Ramos died of heart failure in August.
Ms Reston, 21, was 1.72m (5ft 8in) and weighed just over 38kg (6st) at her death after sticking to a diet of just tomatoes and apples.
A January deadline has been imposed so the new rules are agreed in time for Milan's fashion week. Italian youth minister Giovanna Melandri urged the fashion industry to comply with the changes.
She said: 'The Camera della Moda will take action against designers who do not respect the manifesto. They could be removed from the fashion calendar or, in the most serious cases, banned from the fashion week.'
Flaminia Spadone, an aide to Ms Melandri, said: 'In the Third World, if someone has an index of less than 18.5 they send in humanitarian aid.'
The change is part of a growing international movement to deal with ultra-thin models and the influence they have on girls and young women.
Madrid's fashion week banned models with a BMI below 18 after the death of Ms Ramos, and Brazil and Argentina have joined the campaign. Kate Moss, Naomi Campbell and Claudia Schiffer have BMIs below 18.