This summer saw the fashion industry in a feeding frenzy over a desirable new model - the plumptious, voluptuous Sophie Dahl. Natasha Walter met the 40-30-40 phenomenon who could herald

"Hello!" Sophie Dahl shouts enthusiastically at me as I walk into the cold east London studio where she's sitting in her curlers, surrounded by stylist and hairdresser and make-up artist. Smoking, dabbing nail-polish remover on her scrubby little toes, with her plump legs crossed at the thigh and a couple of angry red spots on her chin, the new cover girl is looking like a reasonably attractive fifth former.

Sophie DahI, dubbed 'darling of the moment' and 'this year's hottest model', talks all the time in a breathy rush. It's not long before the tumbling stream of anecdotes that I interrupted picks up speed again. "I got to this party and I was really bored and he came foxing up to me and said what's your problem and I said I'm really bored and he said what would make you unbored and he came up with these suggestions and said how about a snog so he held me against the wall and gave me this passionate kiss and I said why is that girl staring at me and he said that's my girlfriend and I said so why are you snogging me and he said cos I wanted to, all right..."

Sophie's high voice goes on and on and on. For a while I interview her, but when it's time for her face to be done, I just leave the tape recorder on beside her. When I get home and play it back, the voice doesn't stop for a minute, even though her lips are being painted and her face powdered. She's still amazed to find herself lionised at cool parties, still amazed to find that starchasers want to snog her, still amazed to find herself complimented by Karl Lagerfeld and Ellen von Unwerth.

And she loves talking about it.

Then she goes off into a side room to get into her first outfit, and when she comes back, it's my turn to be amazed. Because this squeaky-voiced 19-year-old has the body of a 1950s screen siren - only more so. Her hair is still in curlers, but her body, her 6ft, fleshy, curving body, is dressed in black lace under-wear and a tight skirt and high-heeled sandals. You see a voluptuous star, where before you were talking to a schoolgirl. She poses like Marilyn Monroe in the famous shot by Eve Arnold, in front of the mirror with her legs slightly apart and tier skirt tugged up. Her huge breasts and rounded stomach and soft haunches push against the clothes. She staggers a bit in her high heels, hut she makes a convincing Marilyn Monroe; even the breathy, childish voice is Marilyn's, and she obviously loves posing; she pushes her chest out and wiggles her bum to the music and makes sexy "moues " at her own reflection.

It's unique to see a model who is as striking and sexual a physical presence as Sophie DahI. Apart from anything else, there's a hell of a lot more of her than there is of most models

Forget Kate Moss's dinky 33-24-34 body. Sophie tells me she doesn't know her measurements, but later her agency makes her measure herself for the casting of a U 2 video, and she discovers she's 40-30-40. 'What's more, she doesn't have the hard-bodied look of women like Pamela Anderson whose rubbery breasts perch on a taut little body. Her flesh does all the things that flesh does naturally; it quivers and dimples and looks soft and pillowy around her bra straps and waistband.

The photographer, Stephanie Rushton, is delighted with her. Stephanie used to work for David Bailey and then moved into advertising and portraiture. "For a time I avoided fashion photography," she says, seriously. "I didn't want to be implicated in making young women feel unhappy about themselves. A lot of the models you see aren't happy with themselves, and their agencies keep telling them to lose weight."

For the next shot, Sophie lounges on one of those inflatable armchairs that you find in smart studios, and when she isn't pouting, pushing her face into a would-be seductive pose, she relaxes, and then you see a different face. There's something cat-like about it, something knowing. She looks much older than her 19 years and, suddenly, beautiful.

Leafing through her 'book', the catalogue of her modelling career (all of five months, from her first sitting for Nick Knight to her shoots with Karl Lagerfeld for German "Vogue" and David LaChapelle for "Vanity Fair" I see that unexpected beauty surfacing - but only sometimes. She looks like a funny rabbit in this month's "Elle" and a plump schoolgirl in the "Evening Standard" but in some unpublished portraits, Ellen von Unwerth caught the charmed languor that occasionally descends on her face.

Her body changes, too; it can look sinuous, a dream of curves tapering into finely cut calves or well, lumpy. "There's something really clean and clinical about being thin," she says to me later. "My body isn't like that." In the magazine "i-D" she wrote, "I look a bit pornographic and that scares people." But you're happy the way you are? I ask. "Yes, I am. This morning I got up and looked at myself in the mirror and thought, I rather like my body." Her evident enjoyment of her own beauty is one of the best things about Sophie DahI. "I've got huge bosoms," she tells me proudly, more than once, as though I couldn't see them.

What brought her into modelling with that unfashionable, fleshy body ? You could argue that she's a girl with the right kind of attitude, that attention-seeking self-centredness that models need . Her grandfather was the late Roald DahI, the author who so famously celebrated the joys of chocolate - a lesson that Sophie has not forgotten. And her mother is Tessa DahI, herself a famous beauty whose lovers included Peter Sellers and David Hemming. Sophie was born when Tessa was 19 and they went on to live in 17 different houses in the next 19 years: London, Boston, Martha's Vineyard, London again, New York, an ashram in India, back to London, and so on. Sophie may, by her own admission, have messed up her schooling, but she certainly learned to grab attention where she could.

In all the stories she tells about herself -stories about going to a rave dressed in Gucci satin pants, about making a scene in a Paris airport, about being caught by paparazzi as she was shopping in Safeway, about her new boyfriend, a lonely Russian boy who holds her all night long, about dancing for David Bailey in hold-up stockings and a G-string, she puts herself centre stage. You can see that being photographed is something that she wants, that she revels in.

But, of course, no one can argue for long that modelling is about inner strength - it's the triumph of surfaces. Her sudden arrival on the modelling scene was really down to the usual play of chance; the right pretty face at the right fashion moment. " I'd just had a screaming fight with my mother," she tells me, "and I was crying away on the street and this amazing woman got out of a cab in a Philip Treacy hat and see-through trousers with all these bags. I helped her into her house and she said, 'Why are you crying?' and we sat down on her front doorstep and talked about it all and she said, 'Well, I think you should be a model because you look like a Playboy bunny."' The dice had come up sixes; Sophie's amazing woman is indeed an amazing woman. She turned out to be Isabella Blow: patron of Alexander McQueen, muse of Philip Treacy, "Vogue" stylist, partygoer, fixer.

The first shoot Sophie did was set-up by Blow and Nick Knight, who had long wanted to photograph larger women, and whose pictures of another size 16 woman caused such a hoopla when published in "Vogue". First intended for "Visionaire" magazine, the photographs of Sophie were then republished in "i-D". The dice were really rolling; it was the right time for the entry of a big model - waifs couldn't get any thinner, and fashion thrives on the new. Now, Sophie groans when she looks at the Nick Knight photograph in her book. It's a spread of her naked body with her breasts pointing into the air. "I felt vulnerable," she says sadly, "not during the shoot - but when I saw it published. Nick had blown up my body from the waist down. I'm not really that fat. I think he just saw it as a challenge to th e fashion industry." She might disdain it, but it worked. The challenge was thrown down, and the fashion industry took it up. Sophie was taken on by the very agency, Storm, that launched Kate Moss a few years earlier.

Watching Sophie, you're reminded of so many of the beauties of the past. In front of the mirror, she may be Marilyn. But in a strapless black dress, twining her white arms round a pillar, she's Anita Ekberg in "La Dolce Vita"; in a tight suit with her hair pinned up she's Kim Novak in "Vertigo" right down to that tense gaze and the slightly staggering walk in her high heels. By this time Sophie DahI has stomach ache, and if she's looking tense, it's because she's in pain. "I always get stomach ache the day after I have sex, she tells us all. "You should drink cranberry juice," says the sympathetic make-up artist.

Given that she looks like such a throwback to the classic beauties, why is her appearance on the modelling scene such a new, even revolutionary thing? For decades, we've only had waifs and gazelles parading through our glossy magazines and fashion has celebrated that. Apart from the odd departure by Vivienne Westwood or Christian Lacroix, it has been miniskirts and slip dresses and skinny trouser suits all the way - not the best clothes for the ins and outs of a woman like Sophie Dahl.

The classic feminist line is that the obsession with the very thin is a masculine conspiracy to keep women quiet and hungry, but that line looks pretty questionable if (like me) you've worked at a women's magazine. "God, she looks fat," mutter the, er, female fashion editors when photographs come in of Christy Turlington with her thighs slightly bigger than her calves. For some reason, women decided they didn't want women with breasts and hips, women who looked sexy and curvy, in their magazines. Alexandra Shulman, the editor of "Vogue" says, "Whenever we put a model with breasts on the cover, the women here say it looks like a men's magazine."

So, while women shaped like Sophie Dahl disappeared from "Vogue" and "Elle" they were still celebrated in "Loaded" and "Razzle". That's a gap that has to be closed, if young women can learn again to see their flesh as cute, not gross; as their own, not just there for men's delight. The time seems right to close that gap; Sophie is getting an enthusiastic response from other women. "I was in a bar last night and these two women were going, is that Sophie DahI, is that Sophie DahI, and then one of them came over to me and said, are you Sophie DahI, because I just want to thank you, that's all, I just want to thank you for what you're doing for women.

Sophie DahI is not Miss Average; on the contrary, her once-in-a-generation combination of fine features, grand stature and voluptuous curves makes her quite as much of a freak as, say, Jodie Kidd, the walking skeleton. But women's sense of optimism at the emergence of this bigger model is not necessarily misplaced. Her picture has been pinned up on the notice board of her little sister's school, because the headmistress thinks she's a good role model. If young girls begin to fantasise about taking up space, about throwing their weight around, what a revolution that would be. Sophie might be a pointer to a new kind of openness in images of beauty. "I don't think we're just going to see a change to lots of big models," says Alexandra Shulman, "but I think we might see different women, of all kinds, women who don't conform to one type of beauty."

Fashion isn't suddenly going to write itself an equal opportunities policy. But I enjoy watching Sophie DahI being photographed as I've never enjoyed a day in a studio before. It's fabulous seeing her stalk around the studio like a soft, padded cat. While other models' bodies tend to suggest punishment - starvation, exercise, even surgery - Sophie's body suggests pleasure: food, drink, sleep, sex.

She finds modelling funny, which it certainly is if you look at it the right way. We've seen it as fraught exploitation for a long time, but with someone like Sophie it's more of a game. "I always enjoyed dressing up and playing with my mother's make- up," she says delightedly. "When I was doing the shoot with David Bailey he kept coming on like the guy in the film Blow-Up, you know, give it to me baby, yeah, sexy baby. He got me to hold my bosoms and dance in these stilettos and G-string. I found myself with my lips trembling, and moaning, like I was turned on. So I started laughing. I laughed and laughed. When I told my mother, she said it was the funniest thing she'd ever heard."

She's too down to earth to play up to her own allure. "I've never been very good at hard to get." She laughs. "Sometimes men like you because they think you're some mysterious creature, you call them up and say, come out for tea, and the next thing you know they are off to Cannes with Naomi Campbell." But she's head over heels in love at the moment with her new boyfriend. "Blissed out," she says happily. "I'm just walking around smiling at people. You know; when all the romantic songs you hear make you t hink, I understand that." She giggles happily, and then pauses. 'And then you get dumped." She sighs. "And you go around thinking, 'I am Tammy Wynette."' For someone so young, Sophie DahI has a good line in homespun wisdom.