On the eve of London Fashion Week, Erin O'Connor – described by Karl Lagerfeld as "one of the best models in the world" – had a confession to make. "I'm a fashion model and I don't fit into the sample sizes. I haven't for some time. At one show I couldn't get into the trousers. The designer said, 'What happened to you?' I replied, 'Why don't you make your trousers bigger?'"

She described herself as being made to feel like a "commodity". "Fashion is built on perpetuating fantasy. There is a sense of uniformity. We have forgotten how to be individuals."

O'Connor was speaking alongside equalities minister Lynne Featherstone at a debate for All Walks Beyond the Catwalk, an ambitious event held over three floors of the National Portrait Gallery. The aim? To stamp out "the dominance of a singular aesthetic", or fashion's obsession with youth, perfection and size zero.

The latest target? Airbrushing and trickery in advertising. Several key players called for industry regulation with various ideas proposed: kite-marking to indicate digital tampering, or a "golden star" system for natural images.

Organiser Caryn Franklin, the one-time BBC Clothes Show presenter, led the attack: "We must question the manipulation of imagery." Many magazine editors have told her, off the record, that they are concerned that the culture of digital enhancement is getting out of control. "They wish there were guidelines for dealing with photographers who ingratiate themselves with celebrities by creating the most polished of shots."

On view for one night only was Rankin's Snapped, a photographic collection of models of all sizes and ages in creations by Vivienne Westwood, Stella McCartney, Matthew Williamson and other British designers, all inspired by portraits in the gallery. An estimated 3,000 visitors turned up, marking the unofficial opening of London Fashion Week, which starts on Friday.

This week, O'Connor also opens the doors of her "models' sanctuary" in London's Covent Garden for the fourth year running. This is a non-profit drop-in centre visited by more than 200 models a day during the shows, where they can seek advice from nutritionists, life coaches and physiotherapists. "Ninety per cent of models are aged 16 to 19. As an industry we have to take responsibility for them," said O'Connor.

On the debate's stage, the predominant colour was regulation fashion black. The only other glimpse of colour came from Featherstone's white shirt and the Christian Louboutin red-soled shoes worn by Elle editor Lorraine Candy. She opposes regulation: "We're not a charity. We're producing what women want. If they didn't want it, they wouldn't buy it."

She said, however, that she had recently rejected "a very famous young Hollywood celeb" for a cover shoot because she had had too much plastic surgery. "It was unrealistic to make my readers feel they should look like that."

Since All Walks was set up in 2009 by O'Connor, Franklin and fashion PR Debra Bourne, more voluptuous models and older women have appeared on the catwalk, most recently at Prada and Louis Vuitton. Tackling digital enhancement is the next step.

"Women believe these images are real," said panel member Kiki Kendrick, the advertising guru who came up with the "size 16 Barbie" for the Body Shop. "That is what happened with the girl who had herself injected with silicone," she added, referring to 20-year-old Briton Claudia Seye Aderotimi, who died in Philadelphia last week after a "buttock enhancement" operation went wrong.

Perhaps the least enthusiastic panel member was the equalities minister. Featherstone said that while she was concerned about "the consequences of this unattainable ideal", the issue could not be about "banning or legislating". Surprising comments perhaps, given that last year she called for the air-brushing kitemark as part of the Lib Dems' Body Confidence campaign.

At the National Portrait Gallery it was a different story: "As both a politician and a parent I can see the kind of pressure that people of all ages are under to look a certain way, and I want to see something done about it. But to change the way things work we can't impose rules from above and demand that magazines and catwalks only feature people who are a certain height or a certain weight."

These remarks come in stark contrast to the line taken by her Lib Dem parliamentary colleague Jo Swinson. She supports regulation, arguing that this is an issue of social responsibility. "The extent to which people's basic body shape is changed is something that is not widely known. I would prefer something that gives the consumer information about how much airbrushing has gone on. One option is a labelling system, a coloured dial like the one used for nutritional information, so you could differentiate between 'purely fantastical' and 'not airbrushed at all'."

She added: "We are getting some traction [with the fashion industry] and the threat of regulation is key. If advertisers refuse, I believe the government should keep an open mind about regulation."

Calls for a practical solution are gaining momentum. An all-party parliamentary group on body image launches next month and a new study on the psychological impact of fashion imagery is expected from Cambridge University next year. Swinson recently submitted a portfolio of 172 academic studies on the detrimental effects of idealistic advertising to the Advertising Standards Authority. It was rejected. Swinson said: "They are resistant to change. They say they don't regulate the portrayal of the body and only react to complaints."