Which are the real Warhols?

Andy Warhol must be chuckling in his grave over a report in Vanity Fair that as many as one in six Warhol works is a fake. According to the report, works that Warhol "discussed or produced in the company of Factory assistants have been denied, despite multiple affidavits from those assistants".

New York is bracing itself for a Warhol washout at next week's sales after a two-year price boom that has seen as much as £4 million change hands for a wallsized Warhol canvas.

Confused? You should be. Warhol deliberately blurred the line as to who made what in the Factory. His estate, a secretive group of purported associates and professional advisers, established an authentication committee in 1995 to judge the status of any works which were submitted to it. The committee issues its verdicts through the estate's lawyer, Ron Spencer, and its assistant secretary, Claudia Defendi.

Its assessment criteria are mysterious and apparently inconsistent. Many works that seem authentic have been returned with the dreaded "not the work of Andy Warhol" certification.

Take pity on Joe Simon, for example. He bought a Warhol self-portrait of apparently impeccable provenance, authenticated by the late Fred Hughes, who was the original executor of Warhol's estate and a trusted associate of the artist's. When Simon came to sell the painting two years ago, a $2 million deal was agreed, subject to authentication. The committee returned his canvas unverified and without explanation.

The same committee has been filtering "new" Warhol originals onto a hungry market. Collectors who paid dearly for their works are now screaming conflict of interest.

The problem arises from the way Warhol worked. His New York studio, known with little irony as the Factory, churned out canvases using the simplest silkscreening techniques, often without Warhol being in the building, or even in the country. In theory, Warhol supposedly approved all works that left the studio, but many are suspected to have been squirrelled out of the studio in the marijuana haze of the late Sixties, to be sold on the side.

These works are physically identical to the certified originals, made in the same way, by the same people, using the same screens and paints. And then there were the paintings that Warhol himself set aside for a rainy day. Charles Saatchi once told me that he bought many paintings directly from Warhol himself, rather than from commercial galleries, visiting him at his Manhattan town house, where classic Warhol canvases lay rolled up under the bed. In this way, Saatchi assembled one of the finest Warhol private collections.

When it comes to designating a work, the law recognises the artist's intention. Warhol's game, however, was to give little of his intention away. He strategically deferred to others around him, seeming almost incapable of making a decision for himself. That was the philosophy that made him famous. The trouble is that art lovers, and art buyers in particular, cling to the idea of a uniquely authored work; that is the only way we can understand what we are looking at.

An American artist, Mike Bidlo, a specialist in copying modern masters, wanted to make fake Warhols, so he borrowed Warhol's original silkscreens to rustle up some new Marilyns. These works, subversive as they are, have been redesignated as the work of another artist. They cost about one hundredth of the price of the so-called original Warhol.