Jack Nicholson loves him. The public adores him. His erotic art has made him millions and his posters outsell Van Gogh and Star Wars. So why is Jack Vettriano so bitter?

Jack Vettriano, former Fife miner, self-taught artist and, despite the derision of snooty critics, perhaps Britain's most commercially successful painter, strides down the drive of his grand, recently purchased Scottish country pile. He is currently "in hiding", and he prefers to keep the address secret.

As he moves, his lank, oiled hair, which frames an unexpectedly earnest face, bobs about his shoulders. At 45, there is nothing now of the miner about Vettriano, whose paintings have made him a millionaire in less than a decade. But then he was never romantic about his working-class roots and, in particular, the five years he spent underground after leaving school at 16, "to become a man, just like my father".

To critics who ask why he doesn't draw on the hard reality of the pits, he says bitterly that if they had been there in the grime, they would understand the escapist allure of his paintings - two of which, Mad Dogs and The Singing Butler, have gone on to become the UK's best-selling posters, dwarfing the sales of other less well- regarded artists, such as Monet and Van Gogh.

Settling into an armchair in his sumptuously decorated period-drawing- room-cum-temporary-studio (a new studio is being built above the garage that will house his collection of classic cars), the man who was raised in a miner's row in Methil, one of Fife's grimmer seaside towns, clearly feels that critical understanding of his work has lamentably been lacking.

For despite his phenomenal popularity, not only with the masses but also with movie stars such as Jack Nicholson (who owns four Vettrianos) and private collectors, Scotland's highbrow commentators, with very few exceptions, dismiss Vettriano's work as "rich man's soft porn".

Complaints about the pornographic nature of Vettriano's work centre on the raw sexual canvases - the bulk of his output - in which men are invariably wolfish (it seems fitting that Nicholson is a fan) and the females red- lipped, hard-faced and vampish; as Vettriano puts it, "verging on the cheap". These works are understandably deemed too racy for the poster circuit.

Meanwhile, the public's apparently insatiable demand for his work seems only to have convinced critics that whatever it is that Vettriano produces, it is not "high art". Does that make him ambivalent about his success? And does he not worry that the sheer ubiquity of his work may undermine demand for the original paintings?

Vettriano lights the latest in a never-ending chain of high-tar cigarettes and looks around a room as deliberately mannered as one of his paintings. His own manner has a little of the sexy seediness of his canvas characters. "I can't complain that The Singing Butler is the best-selling poster in Britain," he says, in a surprisingly soft voice. "But at the same time I can't bear to see it too often. Don't think it doesn't haunt me.

"I've just been reading Lowry's life story, and about how he took friends' advice not to go down the commercial path. But why shouldn't someone with only pounds 20 to spend be allowed to have an image of mine in their living- room?"

On his own living-room wall hangs a Vettriano original called Game On, in which a man in braces pins a woman in black underwear and silk stockings against the wall with one hand, while the other dips between her parted legs. It was dubbed the sexiest work at this year's London Contemporary Art Fair and sold for pounds 15,000. While Vettriano's fame rests largely on less explicit, postcard-friendly images, it is work like this that attracts private buyers such as Robbie Coltrane, Terence Conran and Tim Rice.

The artist's agent, Tom Hewlitt of London's Portland Gallery, says there is no sign that sales of Vettriano posters and cards are bringing down the value of originals. More than 400 private buyers are waiting to purchase an original, and The Singing Butler was recently sold for pounds 33,000.

Still, Vettriano seems less comfortable with his rampant commercial success than Hewlitt. "I think it's nice and god-awful at the same time," he says, though he admits he has no plans to curtail the poster sales, not least because they are reported to bring him at least pounds 150,000 a year in royalties. More than money in the bank, however, public demand is balm on his wounds.

Only a few miles of water separate Edinburgh from Fife, but in that expanse a cultural chasm yawns between the capital's cultural elite and the mining- turned-factory working classes from which Vettriano springs. Vettriano remembers moving to Edinburgh, too shy and gauche to venture into smaller art galleries, uncertain about what gave a man the right to call himself an artist.

As a new generation from Glasgow School of Art (including Peter Howson and Ken Currie) was emerging to ecstatic acclaim, Vettriano, who was part of no artistic movement or feted graduate year, was surprised to sell two paintings submitted to his first important show. Within a few years he was to eclipse the Howsons and Curries, but only in terms of fame and fortune. The critical acclaim heaped on them has eluded him.

Duncan Macmillan, professor of art at the University of Edinburgh - who has been dismissed by Vettriano as "Scotland's answer to Brian Sewell" - sums up the prevailing attitude. Vettriano is rejected, he says, because, put quite bluntly, he is no good. "He's welcome to paint, as long as nobody takes him seriously," is his withering analysis. Prof Macmillan is currently updating his history of Scottish art to take in the last decade. Vettriano will not get a mention.

The artist himself, however, believes that the hostility has nothing to do with talent. "I am good at applying paint to canvas," he says. "It is the subject matter - sex - that they don't like."

For those unfamiliar with Vettriano's brooding canvases, they are populated with characters he labels "malcontents" - men and women so sexually driven that domesticity is an impossible compromise, who play out the mating game against a backdrop of candlelit bedrooms and back-street bars. The women are usually in a state of undress, while the men tend to keep their clothes on. Everyone seems to smoke.

Although Hewlitt claims that as many women as men buy Vettriano's work, it is perhaps unsurprising that some women find his paintings offensive. For the Scottish critic Claire Henry they are "easy viewing, voyeuristic", essentially soft porn. But Vettriano - a divorcee and self-confessed malcontent himself - argues that he is portraying life as it really is.

"What infuriates me most is this notion that I paint to titillate," he says. And even if you hate his work, that particular criticism does seem unfair. For while his opinions are contentious, they seem sincerely held. The sexual impulse certainly appears to have been the defining force in Vettriano's own life. He can - and does - talk for hours about sex; his thematic obsession is a bonanza for journalists seeking salacious copy, and another stick with which art circles can beat him. But the painter says defiantly that he won't change to please the critics. "You struggle as an amateur to get confidence in your ideas," he says. "When my first paintings sold, I cannot exaggerate the confidence I gained. I realised people did want what I had to offer, and that I could provide it in spades."

He claims that his own youth - though it had none of the glamour of his paintings - is a bottomless pit of inspiration. The mating ritual - predatory and sometimes brutal - played out each weekend in Fife's dance halls, fairgrounds and beach-side cafes was the foundation for his enduring view of life. "What infuriates me is the sexist label," he says. "I paint life as I see it. I am not here to improve it."

Vettriano's cynicism towards critics extends to the business as a whole. "I used to believe in art movements and all that," he says bitterly. "But now I just believe in power and money." He is scathing of the appointed "guardians of public taste" - curators who are disinclined to buy his work - and he argues that the art market these days is "just a bloody game".

But though he is embittered, his comments do offer some comfort. After all, he jokes, if it really is only a bloody game, his "reassessment" is probably just around the corner. Jack Vettriano recently bought a studio in London, where he hopes critics will prove to be more kind.