Focus: Is Kate Moss the new Mona Lisa?

The nation is in thrall to the Kate Moss Code. A severe smile and enigmatic sexuality are captivating, looking out at us from newspapers everywhere. Christie's has just sold a version of them, translated on to canvas in Lucian Freud's Naked Portrait 2002, for £3.9m - the second highest price ever paid for a Lucian Freud. The girl from Croydon has them in equal measure with the mystery woman of the Louvre. George Sand said the 16th-century supermodel we call Mona Lisa possessed la laide séduisante (seductive ugliness). Kate Moss has the same: those thin, endless legs, tight boots, tighter trousers, amazing hair. Her image is everywhere. But Kate Moss is not conventionally beautiful. Dan Brown, author of The Da Vinci Code, says Leonardo painted an androgynous self-portrait. Moss's gamine look has a similar sexual ambivalence. If these are universal women, there is something odd about the conventions of desire. Still, each has something we have all been looking for.

The nation is in thrall to the Kate Moss Code. A severe smile and enigmatic sexuality are captivating, looking out at us from newspapers everywhere. Christie's has just sold a version of them, translated on to canvas in Lucian Freud's Naked Portrait 2002, for £3.9m - the second highest price ever paid for a Lucian Freud. The girl from Croydon has them in equal measure with the mystery woman of the Louvre. George Sand said the 16th-century supermodel we call Mona Lisa possessed la laide séduisante (seductive ugliness). Kate Moss has the same: those thin, endless legs, tight boots, tighter trousers, amazing hair. Her image is everywhere. But Kate Moss is not conventionally beautiful. Dan Brown, author of The Da Vinci Code, says Leonardo painted an androgynous self-portrait. Moss's gamine look has a similar sexual ambivalence. If these are universal women, there is something odd about the conventions of desire. Still, each has something we have all been looking for.

Kate Moss and Madonna Lisa - "Mona" is a diminutive - have much in common. Great celebrity for a start. One of Andy Warhol's first commentaries on the mysteries of fame and the mass production of imagery gave Leonardo's sitter the treatment early on in the 1963 silk-screen print, Thirty Are Better Than One. Kate Moss has done very well to spin out her 15 minutes into more than 10 years. La Gioconda has done it for five centuries.

And then there is commercialisation, the celebrity endorsement of products which is one of the great compensations for fame. Here Kate Moss has done better than her more inscrutable elder sister. Moss's sexually ambiguous figure has illuminated countless full-page ads and billboard poster sites. The Mona Lisa's image was first exploited commercially in 1915 when an Italian laxative manufacturer launched Gioconda Acqua Purgativa. Her mysterious dignity, however, survived; possibly even enforced by the incongruity. Italian names sometimes describe attributes: gioconda (smiling) is like our "jocund". Perhaps her expression had a gastro-intestinal source. What else could explain Marcel Duchamp's 1919 version of the Mona Lisa bearing the derisive legend "LHOOQ", pronounced elle a chaud au cul - "she's got a hot arse".

But let's not forget the Freuds. Sigmund was fascinated by Leonardo and made him the subject of a bizarre 1910 essay about psychosexuality whose thesis (a complicated ramble about female phalluses, birds' tails and repressed homosexuality) was based on a mistranslation of a recalled Leonardo dream, something Freud acknowledged in 1923. But this did nothing to diminish the painting's mysteries. On the contrary, it only added to the effect started by art historian Giorgio Vasari. This Florentine gossip (who never actually saw the painting) had no inhibitions in claiming that if you examined it very intensely you could feel the pulse in the throat.

We have similarly intimate access to Kate Moss thanks to our local Freud, the psychoanalyst's grandson, Lucian. Last Sunday he could be seen in Christie's, South Kensington, an ancient elfin satyr in a big coat and scruffy shoes. It was a preview of a modern art sale. His huge nude of Kate Moss is a crude daub of little quality which wholly fails to capture the sitter's ambivalent allure. Still, examining very closely that fold of flesh just beneath the waist, you could, if you follow, almost feel the pulse. The secrets of these women beg, even demand, to be exposed as each Freud so humourlessly did. With Moss as with Mona, only one thing happens to disbelief: it gets suspended.

There is a strong erotic character in the Mona-Moss axis. We owe the identification of Leonardo's sitter to the garrulous Vasari, who said that she was the wife of a silk merchant, Francesco del Giocondo. Modern scholars agree that she was born Lisa Gherardini in 1479. What is certain is that the sitting for the portrait was exceptional. Never mind Moss's hair and make-up; this sitting lasted four years and Leonardo went to exceptional efforts to keep her entertained: musicians and jugglers were hired to "chase away", in Vasari's words, "the melancholy painters usually give to portraits". This may account for her extraordinary expression, described by Angus Trumble in A Brief History of the Smile last year as "the single most famous representation of smiling in our culture".

Bruce Weber and Herb Ritts have photographed a naked Kate Moss, almost a heresy for an international supermodel, but an erotically beguiling one. She retains a hauteur despite Freud's pitiless account of her as "green meat" and the photographer's sharp focus on jailbait nipples and a bonsai bush.

Women of mystery we want to strip: a century after Leonardo, pirate versions of the nude Gioconda started to appear. The connoisseur Bernard Berenson said La Gioconda had a "hostile superiority". Maybe there are paparazzi who think the same of Kate.

Torrents of speculation have accompanied Mona Lisa's journey through history, just as they follow La Moss through the tabs. The Renaissance historian Jules Michelet spoke of the former's diabolical power. Then his son-in-law, the critic Alfred Dumesnil, described "the treacherous attraction of a sick soul ... a soft look, that, like the sea devours". Well, there are impressionable rock stars who feel the same.

In the painting, it is the contrast between Mona Lisa's illusionistic realism and her unattainability that excites. The French romantic traveller Théophile Gautier said: "And you discover that your melancholy springs from the fact that [she] received, three centuries ago, the confession of your love with the same mocking smile she still wears today."

Ah, yes. There is something in Kate Moss that mocks. And just as Mona Lisa's power depends on a sexual character that is both explicit and unknowably subtle, so too does Moss's. This quality disturbs those of insecure sexuality, including John Ruskin, who hated the Mona Lisa, her "archaic smile" exciting memories of feelings he had long buried. His criticism inspired the most famous lines about Leonardo's masterpiece from the Oxford don Walter Pater: "She is older than the rocks among which she sits; like a vampire, she has been dead many times, and learned the secrets of the grave."

That may be what is happening in Kate Moss's current relationship.

Leonardo liked his Mona Lisa so much he never actually delivered it to his client, the sitter's husband, but took the panel to France (where he died) and gave it to François I so that Dan Brown could discover it in the Louvre.

Leonardo's Mona Lisa was unattainable and remains irresistible. Kate Moss has a similar magic: that mocking smile that acknowledges we don't know what. Leonardo's technique depended on sfumato: he painted shadows rather than details.

Kate Moss is famous, but mysterious as well. Walter Pater described his supermodel as "expressive of what in a thousand years men had come to desire". That will do for Kate from Croydon too.

... and what other women think

Tracey Emin, artist

I hate that [Lucian Freud] painting. It's a bad painting. It's ugly and it doesn't look like Kate. Kate's an icon. It's her look, like Twiggy had a look. Kate is the antithesis of what the supermodels were supposed to be. They're always trying to find the new Kate but they can't: she's unique.

Ellie Crompton, style editor, Heat

Kate Moss has never been "papped" looking rough in a newsagent's, she's never been seen carting supermarket shopping bags around and she wouldn't be caught dead in a Juicy Couture tracksuit. This and the distance she preserves from the media are what keep us drooling.

Amy Lamé, comedian and model with Uglies agency

I think she's bought the portrait herself. A lesser mortal might be quite freaked out by it. She can sell anything from Rimmel to Chanel, look totally elegant and classic, Bardot-like, or like a punky London girl. She's a chameleon.

Kate Mosse, novelist, co-founder and director of the Orange Prize for fiction

I've always wanted someone to organise a fashion shoot with "the other Kate Mosse". It would be a blow for older and shorter women. I'm a bit of a fan, actually. She just gets on and does her own thing, and that's great.

Jenny Colgan, novelist

I love Kate Moss and I don't know why. While the rest of us are trying to balance work and men and children and feeling horribly insecure, she makes it all look like a bit of a laugh. I would truly love, just once, to throw on something casually fabulous, toss back a pint of champagne and walk into a party as Kate.

Susannah Frankel, fashion editor, The Independent

Kate Moss is a modern-day Mona Lisa. Although at times she might appear to be featured everywhere, no one knows what she's really like or ever seems to tire of guessing.

Interviews by Katy Guest