I saw two black models and two Asian models during the whole of Milan fashion week - compared with hundreds of white ones. They were the same four I saw in New York. And in Paris it has been even worse. It is absolutely the norm for a show to be entirely Caucasian. If a black or Asian model is used it is either in a streetwear show, such as Babyphat in New York, or as a form of what the London-based Nigerian designer Duro Olowu calls "tokenism - you know, just the one black girl, and she's usually wearing the crazy printed dress". A good example of this was at the Dior show in Paris this week when the Asian model was assigned the Chinese-style dress. "Then these designers claim they get their ideas from the street or from Africa! It's crazy!" says Olowu. "This is just not reflective of our time."

According to targetmarketnews.com black women alone spend £10bn on fashion every year, and that isn't even accounting for the increasingly high-spending Asian markets. But not even the recent launches of Chinese and Indian Vogue seem to have made much of an impact on the runways, although a recent report in the fashion trade journal Women's Wear Daily claimed that there is "a notable increase" in Asian models, doubtless thanks to their naturally slim build and a lucrative market. Of black models, however, there is a definite "lack". This seems ironic considering that, in the past, the fashion industry prided itself on breaking barriers, appreciating all manner of female beauty. In her recently published autobiography, the Sudanese model Alek Wek claims the fashion industry has been nothing but supportive of her, celebrating her non-Caucasian looks and, for the first time in her life, "making her feel beautiful". But Wek's experience appears to be very much the exception to the rule.

"Look at the runways of the 70s and 80s - there was a cross-section and it was reflective of the industry being international. Now you just have white girls who are 14 or 15 and who barely know how to walk," says Olowu.

"I guess the fashion tide has turned - the trend seems to be for young models from eastern Europe," suggests Edward Enniful, contributing editor to US Vogue.

The truth is, though, that fashion has never been particularly strong in this area. Yes, Yves Saint Laurent may have made what was then an extraordinary step by casting black models in his early shows. However, in a book published last year about Saint Laurent, The Beautiful Fall, writer Alicia Drake recounts how the designer once refused to cast a black model because she was too "Musée de l'Homme [Natural History Museum]". At a recent New York panel discussion on the lack of black models, Naomi Campbell recounted how, at the height of the supermodel era, Christy Turlington told Dolce & Gabbana: "If you don't use Naomi, you don't get us." By "us" she meant herself and Linda Evangelista.

During the same panel discussion Bethann Hardison, a former model, said: "This is the one industry that still has the freedom to refer to people by their colour and reject them. I came up in the 60s. I feel it's worse than it's ever been." Such a feeling stands up to closer analysis: there have certainly been black supermodels in the past - Iman, Tyra Banks and, of course, Naomi Campbell. Now, however, it's difficult to name a single black model, let alone a supermodel.

The situation has worsened because the fashion industry has become more corporate, with big conglomerates buying small companies and more money up for grabs, and the whole business has become much more conservative, hence the endless recycling of trends from decades past, and the way the runways look more and more alike - and more and more white.

Who exactly is to blame for the lack of black faces is more of a chicken-and-egg issue. Do the agents ask only for Caucasian girls, or is it the designers? Many blame the former. Julia, a 19-year-old black model who featured in Olowu's show in London last month, and who did not want to give her surname, says that her agency in Paris told her not to bother even coming for the shows this year. "They said I'm not tall enough," she says, with a sceptical roll of her eyes. "I know girls just as tall as me who are going. There is a problem. I've seen it first hand."

Enniful echoes this point: "Casting agencies just don't look for black girls - I've had to start scouting [for non-white models] myself instead of waiting for the agencies to come forward. There are one or two bookers out there who are good, but there definitely need to be more."

But the truth is, casting agents work for designers and if designers wanted something other than Caucasian faces, they would find and supply it for them. Olowu, whose recent show featured almost wholly non-white models, is uncompromising: "The fault lies with the designers - their ignorance and their racism. Yes, it's true that a lot of agencies don't bother sending non-white models - my casting agent told me that I'm the only one who asks specifically for non-white models - but things will only change if the designers take a stand and ask for them."

But it's worth asking if we, the public, play a part. As with the debate about why designers favour models with visible bones instead of flesh, they are showing what they think the public wants. "What's really shocking to me is that no one seems to talk about this problem. The British Fashion Council and the media get all excited about underweight models but on this subject, they don't seem to notice," says Olowu.

"I really can't see anything changing soon," says Roberts. "When I started in this business my being black was odd, but being a man was more so. To be honest, I don't know if things are worse but it's strange now that there are hardly any major black models."

If the public really does want the situation to change, people have to make the point the only way the fashion business understands: by not buying products from designers who believe there is no beauty beyond the pale.