Always suspected conceptual art was a bit of a hoax? It is now - spoofing with attitude is taking off all over. John Windsor reports
Among the prostitutes' cards in the telephone box is one advertising the services of "Peaky" Munro. The photograph shows a portly man wearing potholing gear, including helmet with lamp. He offers "friction, laybacks, slings and hand jams" ranging from "very severe" to "extreme" and there is a telephone number.

In Piccadilly, fixed to the side of the bowl of the Eros fountain, is a bath tap. Beside it, a bar of soap and a towel. Perspiring tourists regard it quizzically. In Leeds, 13 art students arrange for visitors to their exhibition, "Going Places", to be driven by coach to the airport, where the students appear sporting tans and claim to have blown pounds 1,126 of their student union's money on a conceptual artwork consisting of a jaunt to the Costa del Sol. It's another example of spoof art. The trip to the sun was a hoax. So was the tap. And so, regrettably, is "Peaky" Munro.

These days, whoever says conceptual art is a con, had better watch out. Groups of young artists have turned the practical joke into a form of street art that challenges preconceptions about what art should be.

Even the traditional "white cube" is no longer a refuge. Spoof artists hate galleries as much as they hate the art market - except when it is paying them. Unsuspecting visitors to the spoof Gallerie (sic) Winner in Underwood Street, Islington, last month, were ushered into a specially constructed white cube, ceiling and all, within the cavernous space occupied by Bank - a spoof art group.

Lurking behind the reception desk, a forbidding, bunker-like construction made of concrete, was 33-year-old Wayne Winner, a contemporary of Damien Hirst at Goldsmiths College, wearing lounge suit and thick-rimmed spectacles, in the guise of a sharky gallery owner.

Prosperous-looking visitors were shamelessly fawned upon. Nerds were intimidated: Winner even refused to tell them the way to the toilet. He followed them round the gallery smoking a big cigar, glowering as they viewed the artworks - his own - consisting of "Art Protectors", exquisite paintings (so he said) almost completely hidden behind protective steel shields, with titles such as Rabid, Maniac, Women Are Men and Contaminator, priced pounds 1,500 each.

Grown-ups who should be better-behaved have been getting in on spoof art in a big way. The multinational P Bonk & Co is an elaborate hoax, a fictitious Finnish company that has supposedly become a global industrial giant following the discovery by its founder in 1893 of a way of harnessing the power of anchovy oil. The company is now the leader in Localised Black Hole (LBH) technology.

An exhibition featuring Bonk's Dr Yes Cosmic Therapy Unit, historic photographs and contemporary advertising art lauding Bonk products such as Chateau Bonk wine and Bonk's Bermuda Triangles - high-density anchovy briquets aimed at the gay market - is at the National Museum of Photography Film and Television in Bradford.

It is, of course, a satire on the pompous confabulations of company histories that multinationals use to boost their prestige. Spoof art is seldom just a jape. There is usually a message tied to the brick.

Ironically, the Helsinki-based group of four who dreamed up the spoof, including the British film-maker Richard Stanley, have found that it has taken off as a little industry in its own right - Bonk Business Inc.

In Sunderland, you can see the headquarters of another spoof organisation, The Centre For Low Motivation. Its name, in Foamex letters, is on a 40ft board on a disused shopfront, the work of Will Bradley, a 29-year-old graduate of Glasgow School of Art. He says: "If people begin to wonder what might go on in such a place, and whether we have a problem with motivation, then the centre has already begun to exist in their minds."

As part of the Northern Gallery for Contemporary Art's show, "Fact of Life", Bradley has also donated to Sunderland Library 13 books with outrageous titles such as The Hollow Earth, which claims that the centre of the earth is inhabited - just to see them classified as non-fiction under the Dewey system.

Most elaborate art spoof: this year's publishing hoax on 1 April, that had glitterati flocking to the launch in New York of William Boyd's biography of Nat Tate - an artist who never existed. The book was published by the part-time art critic, David Bowie. Artworks by Tate and photographs from his childhood were concocted for the opening bash, which was hosted by the blissfully unaware kitsch sculptor, Jeff Koons. "Yes, I knew Tate," said some. Ha ha.

One of the prime targets of spoof art is the press - those people who can't tell an artwork from a pile of bricks and who keep wittering on about taxpayers' money. Sex-for-art is as good a bait for hacks as holidays- as-art. Angela Marshall, the artist who stipulated sexual consummation with the buyers of her paintings at the Decima Gallery in Bermondsey, south London, in April, turned out to be an imposter. Then the real Ms Marshall turned up and proceeded to do the business.

Well, what did all it mean? One group that uses spoof art to get its message across is FAT of Clerkenwell, a young architectural practice. The letters stand for Fashion Architecture Taste. It was FAT that stuck the tap on the Eros fountain. FAT also put a light switch on a lamp post in Piccadilly and fixed a bottle of whisky with an optic to railings at Oxford Circus. The bottle was soon emptied.

The group seems to share the spoofers' contempt for galleries. They peppered artworks at the Royal Academy's summer show with red "sold" dots, to neutralise the cachet that official spots carry. And they seem to have a fixation with London bus shelters. They thatched one, turned others into art galleries and one into the Karaoke Kong club, complete with illuminated board and a volunteer in gorilla outfit.

The purpose, says FAT's director, Sam Jacob, 28, "is to challenge ideas about what public space is for. It's limited at the moment. You're only allowed to behave in certain ways, to do certain things. We want to extend that." Fat has converted interiors for homes, offices and leisure spaces - but wants to get its teeth into urban outdoor projects.

Art is the perfect medium for teasing established views because it succeeds by tricking the eye. Two-dimensional paint and canvas is made to give the illusion of three dimensions. As Magritte said: "Ceci n'est pas une pipe." And as the young Picasso famously put it: "All art is a lie, but it is a lie that tells the truth." The step from optical illusion to practical joke is but a small one.

The spoof prostitutes' cards in telephone boxes - 250 each of 20 different designs - aped the typography and graphics of genuine cards so accurately that at first glance they were indistinguishable. But each spoof card had stamped on the back "Auto Erotica" and Monitor Arts and Media, the north London collective behind the wheeze.

Only the incongruousness of "Peaky" Munro's potholing preferences, a tuft of what appeared to be pubic hair on a card with no telephone number, or the accusatory card, "How many magazines have you hid under your mattress?" gave the game away.

One card displayed a clip from the cover of a women's romance magazine, listing its contents, including: "Sex at sea: Confessions from the Love Boat." Was that any less of a fantasy than the genuine card next to it, "New New New Expelled Public Schoolgirl Needs Firm Hand"? And how many telephone box users would previously have credited prostitutes with turning urban spaces into art galleries?

Anna Landucci, 26-year-old managing director of Monitor, which raises funds by placing graduates' art with companies, says: "The cards blurred definitions. That was part of the fun."

Martin Creed, 29-year-old Slade graduate, is famous for his multiples - balls of screwed-up A4 paper that he sells with certificates of authenticity and edition numbers for pounds 10 each. The reason he invented them was genuine enough. He was short of money. They are, of course, a dig at the art market - the way that authentication and numbering of editions by a bona fide art graduate can create value irrespective of artistic merit. But the spoof has a twist. They are well-made balls - he rejects four out of five - and connoisseurs can tell a Creed ball at a glance. What's more, they have doubled in price.

Last year, at his show at the elite Victoria Miro Gallery in London, he screwed a rubber doorstop to the floor so the gallery's front door would open only 45 degrees. It made one visitor so angry that he forced the door and broke the doorstop. You can buy the doorstop artwork, Work No 115, complete with subtitle/instructions, "a doorstop fixed to a floor to let a door open only 45 degrees", for pounds 500 in an edition of 10. Creed has already sold four.

So far, there have been no takers, at pounds 2,000, for a previous work of his, Work No 95, subtitled "all the sounds in a gallery amplified". He tried it out at an opening at the Marc Jencou gallery, connecting microphones from the telephones, faxes - and lavatories - to a mixing deck and turning up the volume.

Creed says: "What I know is that I want to make things that communicate with people. I want to explain myself. I want to be loved like everybody else. Beyond that, I don't know. There's nothing particular I have to say.

"So often, my work comes about because I can't think what to do. I have no basis on which to make decisions. I don't want to work in new materials, so I make use of what is already there, like a doorstop, or sound. I'm giving you what's there but in a new form. I want to make something happen. And it does."

His latest work, Work No 200, subtitled "half the air in a given space", involves collecting in balloons half the volume of air in, say, an art gallery, then putting the balloons in the space. This year, in the Analix gallery in Geneva, the 15,000 white balloons were 10ft deep. At Gavin Brown's Enterprise in New York, the balloons were multi-coloured and 14ft deep. Being immersed in them is an eerie experience. You cannot see for balloons, but every movement makes contact, through balloons, with someone else. Creed has sold two of this work at pounds 5,000 each.

The spoof-art group Bank parodied the Rupert Murdoch media empire by making videos of the goings-on at six venues, including its own, where a woman sat in a bath of coffee for three hours, then playing back the accumulated footage at all the venues the following day - as if it were scheduled television programmes. Participants included the ICA, the Transmission Gallery, Glasgow, and the Eagle pub in the City Road, London, where regulars noted nothing out of the ordinary.

Bank seems to be permanently immersed in its own irony. The opening night of the video project consisted of up to 600 people cramming into Bank's HQ in Underwood Street, where the refreshments were dyed white and there were stages with nothing exhibited on them. People drank Beck's beer, gossiped, then left. John Russell, 34, a Central St Martin's graduate and one of Bank's four directors, explained: "It was an empty media event."

The group's publication, The Bank, originally part of the video project, is a parody of the Daily Mirror, and specialises in in-jokes about the young British art scene.

Reality caught up with Bank last year when they staged "God", a show that included a spoof politically correct, multiracial sculpture of the Crucifixion - made from body casts of an Asian woman, an Afro-Caribbean and a Caucasian male. It was gruesomely realistic in the manner of the medieval German painter Mathis Grunewald. Its power transcended spoofery. "My mum is Catholic," says Russell, "and she said the Church should buy it."

Bank (0171-336 6836). FAT (0171-251 6735). Monitor Arts and Media (0171- 700 7388). Bonk Business Inc is on until 28 June at the National Museum of Photography Film and Television in Bradford (01274-727488). 'Fact of Life', Northern Gallery for Contemporary Art, Sunderland (0191-514 1235). Martin Creed, Cabinet Gallery (0171-274 4252).