London - Banksy is the most exciting artist to come out of the UK for more than a decade - or so many people on both sides of the Atlantic will tell you. But is he really so much more than a prankster with a spray can? Guardian art critic Jonathan Jones gives his view. It's not often you hear someone roar the name of an artist as if they were cheering on a football player. In Bristol, however, I once heard a man scream out "Banksyyy!" as he walked past one of his murals. He was in good company. Hollywood, the New Yorker magazine, Sotheby's (which sells him), Damien Hirst (who collects him) and Glastonbury (where he recreated Stonehenge with a group of portable toilets) all concur that Banksy is the artist of our time, the rising star, the news. A poll of 18- to 25-year-olds recently named him an "arts hero" in third place behind Walt Disney and Peter Kay, and ahead of Leonardo da Vinci.

America was originally just a great target for Banksy - but then it unexpectedly took him to heart when he put orange-clad sculptures of Guantánamo prisoners in Disneyland. That was a taster for last year's one-man exhibition in Los Angeles, the opening of which was attended by the likes of Brad Pitt and Angelina Jolie. There were massive queues to see the show's installation of a living room with 18th-century pictures on the walls, containing a live elephant with its body painted pink and gold.

Suddenly Banksy was no longer a merely British obsession. A couple of months ago he got an accolade he could scarcely have dreamed of when he was spraying slogans on walls as a teenager - the New Yorker dedicated a seven-page feature to him. It makes funny reading if you're British: as if describing a journey into some Dickensian slum, the author evokes the seediness and sleaze of the Soho gallery owned by Banksy's dealer - on Greek Street, near some of London's most expensive restaurants.

What are we to make of the Banksy phenomenon? Banksy, obviously, is not his name. You can't help thinking he might have chosen a better tag if he knew he would one day be taken seriously by the art world. I mean ... Banksy. Most people believe that Banksy - who has so far concealed his true name - comes from Bristol or its environs, and his surviving murals in that city have become objects of local pride. A couple of weeks ago it was announced that a new building development in Bristol, instead of destroying his street painting The Mild Mild West, will incorporate it and profit from the association.

We may not know much about Banksy as a person, but we know he's ambitious. He went to Ramallah to paint on the dividing wall in the occupied West Bank, and this summer was booked to enliven the Glastonbury festival. Banksy makes open-air sculptures that are like gags from a Dom Jolyesque television show - he put shark fins in a pond in Victoria Park in east London - and this humour has translated easily into his indoor gallery installations. The resulting stardom must surely soon make anonymity impossible.

One anecdote he does tell about his origins is how, when he was painting graffiti as a teenager, he was chased by the police: hiding under a van, he saw a stencil-like plate on its chassis and decided there and then to use stencils to design his street art. That way he could paint faster and elude the law; but this also meant he could paint better, becoming something far more like a proper artist. Banksy's stencil technique is now what makes his style so recognisable, like Andy Warhol's silkscreens.

Banksy is not just a graffitist but a guerrilla conceptualist. His gags have included surreptitiously infiltrating his own works into museums - the British Museum took a full eight days to notice his chunk of "rock art" depicting a stone age hunter with a shopping trolley, together with the caption crediting it to "Banksyus Maximus" - and has also redone Monet's water garden with a supermarket trolley and bollards. I know you're laughing. Now you've stopped. My favourite is the parody of Andy Warhol he put in New York's MoMA, depicting a can of Tesco Value cream of tomato soup.

What is it that constitutes Banksy's appeal? First of all, he is talented - for a graffiti artist. That's a big qualification. Banksy is fascinated by trompe l'oeil - the art of deceiving the eye - and has quoted from "a man in the pub" a story about art and illusion that in fact comes from the writings of Pliny the Elder. Two painters compete to fool the eye: one paints realistically enough to deceive birds, but the other fools humans. Banksy's TV set would only fool myopic birds. But you get the point: it's far more ambitious and lucid than the graffiti around it. Banksy's stencil method permits him to paint pictures where others just spray their names. It also encourages the use of icons and stereotypes, making his art a long series of variations on themes - and drawing comparison with Warhol from those who see him as a great modern iconographer.

He deprives his art of the qualities that graffiti can offer modern art: its violence and chaos and paranoid mania. The French artist Jean Dubuffet argued 50 years ago that high art was exhausted, and acclaimed graffiti as art brut, raw art. While Dubuffet was admiring graffiti, Jackson Pollock and Cy Twombly were being influenced by it. In 1980s New York, Jean-Michel Basquiat was seen as a raw hero of authentic street art, revitalising American painting.

But Basquiat's art exposes Banksy's. Where Basquiat's has the dirt and mystique of true graffiti, dredging something from Down There, even though Basquiat actually came from a middle-class background and briefly attended a school for gifted children, Banksy is merely one of the lads, having a laugh.

And yet this isn't about talent or lack of talent. One of Banksy's most irritating attributes is his conservatism, as an artist who seems proud of the fact that he "draws", rather than just making "concepts". He appeals to people who hate the Turner prize. It's art for people who think that artists are charlatans. This is what most people think, so Banksy is truly a popular creation: a great British commonsense antidote to all that snobby pretentious art that real people can't understand. Yet to put your painting in a public place and make this demand on attention while putting so little thought into it reveals a laziness in the roots of your being.

Actually, it's fine to like him so long as you don't kid yourself that this is "art" - and you don't believe that for one second, do you? Sotheby's well-educated connoisseurs surely don't believe it either. Collectors presumably do, so the joke's on them. Perhaps the rise of Banksy is the fall of Art - that is, the waning of art as the force it has been in recent culture. A decade ago, the art of the Damien Hirst generation pushed itself into anyone's view of what was happening in Britain. Probably the rise of Banksy means that moment is coming to an end; people care more about other things. Bansky is a background artist, as in background music: like all graffiti, his is essentially an accompaniment to other activities. The reason to admire Damien Hirst is that he makes art as if art mattered. In Banksy, the philistines are getting their revenge.