No, it's not Banksy...

Banksy was unavailable for comment, but a source close to him was as cagey as the Tate's curator, first denying that he'd been approached and then adding, perhaps a little more truthfully: "Isn't the exhibition sponsored by Nissan? Street Art is not supposed to be sponsored."

Banksy's absence does not reflect well on the museum or the artist. The Tate's first ever Street Art exhibition should undoubtedly have featured Britain's world-renowned leading artist of that genre.

Banksy, with his unprecedented public profile, his use of the British media and his commercial ventures - like the exhibition he held in Los Angeles where he sold millions of dollars of his art to Hollywood celebs, more on which later - has no good reason not to play ball with a public institution-which has done so much to bring art to a wider public. I'm not Banksy's biggest fan - but I missed him.

Instead of Britain's spray-can pop artist, there are six foreign exponents of what is a worldwide cultural phenomenon, the second flowering of what used to be called graffiti art and is now known as Street Art.

From left to right, across the Tate's façade, there is Sixeart from Barcelona, aged 33, who creates flat graphic work with bright colours and simple patterns populated by bugeyed creatures. It's South Park meets the Japanese pop artist Murakami meets the Spanish surrealist Joan Miró.

Next along, separated by the Tate's long windows, is 25-year-old French Street Artist JR. He illustrates how radically the American graffiti art of the Seventies and Eighties has been reinvented by a new generation.

Back then, Street Art meant spraying tags, words and images on subway trains and walls in New York.

Three artists rose to fame, calculatedly painting their designs on the walls around New York's galleries and subsequently getting exhibitions in them - Keith Haring, Jean Michel Basquiat and Kenny Scharf.

Nowadays Street Artists are as likely to use stencils, rollers, photographs and glue as spray-cans. The walls of the city for them have become simply surfaces on which to apply art.

JR's medium is usually blown-up photographic portraits. His most celebrated work to date was a series of mugshots of Palestinians and Israelis hung in pairs along the "wall of separation" the Israelis built to fence off the West Bank.

For the Tate, JR has pasted up a grainy, massively enlarged photograph of a man cradling a video-camera like a Kalashnikov - the camera is now the weapon of urban resistance, the image says.

Also in the line-up are two artists come from S"o Paolo, perhaps the most active centre of Street Art in the world. Here graffiti is tolerated by the authorities, and so the artists can execute their works at their leisure in daylight. Once again it's miles removed from the illegal graffiti of Seventies New York.

Os Gemeos, twin brothers, make meticulously painted and skilfully drawn cartoons, whose characters wear checked trousers and patterned shirts, recalling illustrations in a Maurice Sendak children's book.

The brothers have contributed one of their trademark yellow figures to the Tate.

"When we dream, everything we dream has yellow tones. This is something of ours, myself and my brother," they say rather unrevealingly.

This time their character conceals his face behind a white scarf. They've come incognito, a way perhaps to indicate their ambivalence towards being transported from the favelas of their megacity to a gleaming European art gallery.

Nunca, the other Sao Paolo resident, says the same thing in a different way on his section of brown bricks. His walking figure influenced by Pre-Columbian art, and painted in the red hue popular with indigenous Brazilian tribes, is drinking a cup of tea while treading on a severed hand.

Faile, a New York collective established in 1999, who pioneered f lyposting as Street Art, offer an image which combines a Native American out of an old comic book, behind which float brand logos as if on ripped off posters underneath. It's an aesthetic that itself rips off the Italian artist Mimmo Rotella, who made collages from torn advertising posters in the Fifties.

Finally there is Blu from Barcelona, whose trippy work is usually full of monsters, half-man, half-human, and people removing their heads. For the Tate he's painted an enormous head in profile in which he has drawn various scenes - a domestic living room, a nuclear power station which appears to have suffered an accident, a library.

It's a shame the Tate didn't rope in any of the Street Artists from Berlin, another centre for this activity, and that they didn't devote some of their interior to the subject - if they had they could have shown Blu's technically staggering though somewhat repetitive animations of his drawings across real walls (available at http://www.vimeo.com/993998).

There may be no Banksys at the Tate but there is an intense Banksy experience to be had nearby in a tunnel running underneath Waterloo train station on Leake Street.

Here, there are literally hundreds of stencils, posters, paintings, graffiti and tags on view - not all by Banksy but the remains of his Can Festival on the first May bank holiday weekend, which drew an astonishing 28,000 visitors in three days (a popular Tate Modern show reaches that figure in two to three weeks).

It's a remarkable, historic site which should immediately be given Grade I listed status. Behind or around the stencils the walls are covered in dripping paint, marks and scuffs - the kind of textures that many studio artists spend weeks trying to create on canvas.

There are rabbits in shorts, flying toasters, buddhas, children, angels bearing placards saying "Go to Hell", men imprisoned in upturned shopping trolleys, and on the pavement someone's drawn a Scalextric track with model cars whizzing around on it.

Banksy's innovation in Street Art has been to create this storytelling, trompe l'oeil stencil style. His cute children with balloons turn derelict sites into playgrounds and the forces of law and order are mocked (by stencilling, for instance, "What are you looking at?" on the wall in front of a security camera).

It's funny political satire - Hogarth does graffiti - with an ongoing narrative in which the kids rewrite the laws of the land ("By order of the national Highways Agency: this wall is a designated Graffiti Area"), and there are tons of art-historical references, often to Warhol. But it's more sentimental than it is revolutionary, one reason why it has caught the popular imagination

Today the Banksy phenomenon has become so market-driven that it's difficult to assess the art itself.

His collaboration with Damien Hirst - a spot painting lifted up in one corner in illusionistic fashion by one of Banksy's stencil characters - went for almost £1 million at a Sotheby's charity auction last November.

And the prices are almost as daft when the money's not going to a good cause. I went to Bonhams' Urban Art auction last February and watched Banksy's portrait of Kate Moss in the style of Warhol's Marilyn fetch £100,000, even though it was a print in an edition of 50.

Behind me was a woman in a gorgeous pink outfit talking on the phone to someone called Steve. I put two and two together and guessed she was talking to Banksy's dealer Steve Lazarides.

I kept trying to catch her eye but she knew I was a journalist and was avoiding my gaze. Then, finally, when one Banksy print went for £11,000 she couldn't contain herself - "But it's unsigned," she exclaimed to me in horror. "The guy who bought that doesn't know what he's doing."

Banksy's role in this market is ambiguous. The artist, by all accounts a clever and generous member of the human race, is known to give away large amounts of his profits to charity.

Banksy had nothing to do with the Bonhams auction, his people say, and he has nothing to do with the latest manifestation of the commercialisation of Street Art - an exhibition at Selfridges of work by Banksy and others which will later be auctioned in Shoreditch.

A few years ago, he sold people his prints on his website for a modest £150 - now the same work is traded on eBay by small-time dealers for thousands. Having said that, Banksy is not as innocent as he claims.

He unleashed the whirlwind by staging an exhibition of his work in LA in 2006 and selling it for millions to the Hollywood glitterati. Brad Pitt and Angelina Jolie bought a charming picture depicting Michael Jackson trying to coax young children into a cottage in a spoof on Hansel and Gretel. That was when Banksy's prices took off.

The problem with assessing Banksy is not the amount of money sloshing around but the quality of ideas.

The Dutch landscape paintings with helicopter gunships and CCTV cameras in them, the stencilled gorillas with the slogan "Laugh now, but one day we'll be in charge", the old ladies playing boules with bombs, the Diana banknotes - once upon a time Banksy's cartoons would have been designs on birthday cards on sale in the ICA bookshop or T-shirts sold in Hoxton, not "works of art".

The "Danger radioactivity" sign placed in a pond in St James's Park, the framed works of art snuck into the Tate and the kid apparently lifted aloft by a McDonald's balloon would have been stunts on a TV show like Michael Moore's TV Nation or even Beadle's About.

But the history of modern art has often been the story of "low" art forms raised to the level of fine art by collectors and critics so Banksy cannot be easily dismissed as a witty graphic designer.

Nevertheless, Street Art has limits that are plain to see at Tate Modern. It's basically a blend of sci- fi and fantasy graphic art writ large across the urban landscape.

However wonderful it is, it has no more to do with the art in galleries than do Marvel comics, cartoons and graphic novels. It carries the overly familiar message that the grey and decaying city should be brightened up, or presents a vague sense of urban alienation. There is no self-examination - but plenty of self-pity and self-importance.

The art on the outside walls of the Tate is not going to pass by osmosis into the interior galleries unless the street artists improve their act.

It's telling that people who take this art seriously usually avoid thinking too hard about the subject matter.

Instead they concentrate on the conceptual gestures of the work. A real urban space is transformed into a fictional cartoon realm; the images empower their authors and are symbols of resistance against poverty, oppression and exploitation; the work is popular, anti- capitalist and it's generous-spirited.

You can't criticise it without sounding like a slipperwearing, toffee-nosed art critic. I guess that's why I really don't like it.

Street Art
Tate Modern
Bankside, Holland Street, SE1 9TG