All her material possessions: Sally Ann Lasson gets a tour of supermodel Marie Helvin's wardrobe

Marie Helvin says she has been on a diet all her life but judging from a close look at her, it's surely been worth it. Still fabulous at 57, Helvin is in the process of decluttering her life, and to that end is auctioning off many of her clothes and dresses, including the Yves Saint Laurent Rive Gauche wedding outfit from her 1975 marriage to David Bailey. A photograph included in the lot shows the couple with their witnesses: Tina and Michael Chow, Michael Roberts and Cathy Damon, another supermodel of the day. A little piece of fashion history with an estimated value of £300 -500.

The clothes, a hundred or so items altogether, will be auctioned by Kerry Taylor, the upmarket vintage clothing experts, next week in London. Helvin's wardrobe shares the stage with another fashion legend, Audrey Hepburn, whose estate is selling a large number of her clothes, including an unused wedding dress. Was she worried about being overshadowed by the screen idol? "Not really. She's on the front cover of the catalogue, I'm on the back," Helvin says pragmatically. "And it will get a lot of attention." The show has been previewed in New York and is off to Paris before viewing begins in London. The reasonable estimates may take a hike so close to Christmas.

The Helvin collection includes all of the designers with which she is most readily associated: Azzedine Alaïa elastane dresses from the 1980s, Antony Price cocktail dresses and Bruce Oldfield evening gowns. She was an early devotee of Rifat Ozbek, owning several items from 1986 when he was barely out of his New Romantic phase. Oh, what fashion was then, with draped Turkish pants and gold flamenco skirts. But the supermodel has now become something of a purist . "I am constantly searching for the perfect white shirt," she says. "At my age, I can't wear all these short, tight things anymore. I'd look ridiculous. I like a plain white shirt, a black A-line skirt and some nice shoes. And that's it."

She sustained a back injury recently trying to shimmy into an Antonio Berardi dress, so high fashion may be out for a while. And some of her outfits in the past have been so figure-hugging, that she admits to sometimes cutting herself out of them at the end of an evening. Her favourite item in the auction is an Ossie Clark velvet and chiffon evening gown from the late 1980s. Helvin modelled for Clark in 1975 and remained friends with the designer until his death in 1996; the dress was a special one-off for Marie. "It's an engineering work of art," she says, "because there are no buttons or zips."

She is equally enamoured of lot number 270, a collection of Antony Price eveningwear from the 1980s. There is a picture of Helvin wearing one of the dresses with a very impressive embonpoint. "It's all padding," she says, looking at the photo. "He created a body within a body. The breasts are padded, the hips are padded. Even the butt is padded. He loved the glamour-puss thing."

Still, she's not sorry to see it all go. She says none of the clothes fit her anymore ("I couldn't even get a finger into most of them now") and seems to have no sentimental attachment to things. "I don't have the space, I don't want to hang on to everything. The only things I really want to keep are my books." She hasn't got any personal pictures of herself and the auction includes a huge nude portrait of her and the 1977 Adel Rootstein mannequin, a snip at £250-300. She says, refreshingly, that she does not remain friends with her numerous exes – "They weren't my friends anyway" – with the exception of the notoriously tetchy and David Bailey. As he owns all the copyright to the pictures he took of her, this is probably rather sensible.

Her famous friendship with Jerry Hall is also no longer. She says they just don't have anything in common any more but, of course, all the pap pictures from the 1970s and 1980s are of them together. Here they are at the races; here they are at Club Sept in Paris with Andy and Yves; here they are smoking and hanging out. Anyone who saw them on the catwalk together would not easily have forgotten the sight of these two American girls, having fun, with supermodel bodies that you couldn't quite believe the perfection of even though they were right in front of you. It wasn't just dieting that produced that body, more a genetic anomaly, a fortunate mix of American and Japanese genes. So, she will be happy when she finally retires home to Hawaii? "It will be nice to eat," she says longingly, and confides that once a month she treats herself to a croissant.

She is also looking forward to wearing a "muu muu", which is not a new line by Prada but a sack-like dress designed by missionaries when they arrived in Hawaii and sought to clothe the natives. She didn't like wearing them as a child but now appreciates the appeal of an unconstricting garment. What a long way she's come since her Hervé Léger bandage dress of 1998 (£300-500).

Helvin is the face of a new range of Agent Provocateur couture lingerie which she shot in the summer, just before her 57th birthday. "I didn't want to be 57 when I did those pictures because I thought that was too old to be standing around in panties," she says, "but I'm over the shock now." She has also written and narrated two documentaries for CNBC about Japan and Alaska – "I could not believe that there was such a thing as snow when I was a child in Hawaii. I have always been fascinated by it" – and is hoping to do more.

She published her autobiography last year and is working on a health book but somebody really ought to snap her up to do a clothing range. Like Twiggy before her, Marie Helvin has seeped into the national consciousness. She may share her auction with Hepburn, whose most famous role was Holly Golightly, but Helvin embodies the spirit of that character more precisely in a way. She doesn't own any property, is unattached and has no children. If we allow her to drift back to Hawaii we will miss her more than we know.

The Passion For Fashion auction, including the Audrey Hepburn and Marie Helvin Collections is at Kerry Taylor Auctions on 8 December with viewing from 6 December at La Galleria, 30 Royal Opera Arcade, London, SW1.