A designer's decision to use size 12 and 14 models at London Fashion Week yesterday caused a behind-the-scenes row that ended in his stylist walking out. Mark Fast, known for his sculpted mini-dresses reflecting the recent body-conscious trend, used three models of a size larger than the catwalk is used to seeing – Hayley, Laura and Gwyneth – to model his knitwear designs.

Within an hour of his show on Saturday morning, an Elle magazine journalist sent out a tweet saying Fast's stylist had resigned over his use of the models. Fast's managing director, Amanda May, said she was "so happy we stuck to our guns over the casting". She told the Observer that stylist Daniella Agnelli had stepped in to help out. "She was in the studio until after 2am with Mark perfecting the show. We are really grateful to her."

Fast had been working with size-12 model Hayley Morley for the LFW photography exhibition, All Walks Beyond the Catwalk. It launched on Friday at London's Somerset House with a party attended by Alexandra Shulman, editor of British Vogue, and prime minister's wife Sarah Brown, who admitted she had "sneaked out" of a number 10 reception for British designers she was hosting.

The exhibition features models aged 18 to 65, in sizes 8 to 16, wearing outfits created by young London designers. It aims to change the narrow vision of beauty offered by the fashion world. The size issue is always a sore point within the industry. The 2007 Model Health Inquiry was launched by the British Fashion Council in response to the death from starvation of several models who had been slaves to the size-zero trend. It failed to set out any firm industry guidelines but the debate has gained momentum; this month, plus-size model Crystal Renn launched her autobiography at a glittering Manhattan party and talked of a new vogue for women "lush and sparkly with nary a jutting collarbone in sight".

On Friday Professor Ulrike Schmidt, chair of the Institute of Psychiatry's eating disorders team, spoke out about the dangers of very thin models. "We are very concerned that the lack of medical checks for models at London Fashion Week, coupled with an environment where being underweight is the norm, prevents those with eating disorders from gaining insight into their condition."

Vogue editor Shulman wrote to top designers in June this year, asking for larger-sized clothes for the magazine's photo shoots. "We have now reached the point where many of the sample sizes don't comfortably fit even the established star models," she wrote. Her views were backed by designers such as Roland Mouret. "Back in the 1980s, when supermodels were several sizes larger than top models today, the clothes worked on bigger bodies."

All Walks Beyond the Catwalk aims to change the perception of young designers towards age and weight. Exhibition curator and fashion TV presenter Caryn Franklin said: "Working with designers early in their career to introduce this shift is crucial." In Fast, she may have found her first true convert.

Commentary: 'A reminder of my own prejudices'

When Hayley Morley first strode down the catwalk at Mark Fast's fashion show, I admit I blinked. It was a chastening reminder of my own prejudices, how much I expected to see a certain size of model on the catwalk.

Fast's casting decision was a great one. We do need to see more women who reflect the weight and shape of the rest of us on the catwalk. Their presence genuinely altered my appraisal of the clothes on show, making me consider how I would look in these designs rather than viewing them purely as a reporter. Fast has previously been criticised for producing super-short, super-tight dresses that only the super-thin would feel comfortable in. It was interesting to be shown how wrong that is.

Fast's casting wasn't a total success though. One side-effect of having average-sized women on the catwalk was to emphasise just how thin some of the other models were. The other problem? Purely practical. The tightness and shortness of the dresses was fine. The lack of undergarments was not. Morley's modelling agency, 12+UK, lists her chest size as 34D and that's a bosom that needs support. I doubt Morley would strut down to the shops without a bra – it's a shame she had to walk the catwalk without one.

Cast normal size women again, Fast, it was great. But next time, knit them a bra as well as a dress.

Alice Fisher