One in six 'original' Warhols is a copy

Furious collectors stand to lose millions after the artist's estate reclassifies thousands of prints ascribed to him

If you've just paid £5m for one of Andy Warhol's iconic Marilyn Monroe prints you'd better start searching for the great man's fingermarks. In a move that is alarming pop art collectors around the globe, the Warhol estate has re-classified as many as one in six of the pictures previously attributed to him as copies.

The sweeping reappraisal of one of the 20th century's most prolific artists is based on a strict new definition of what constitutes an original Warhol. The Andy Warhol Authentication Board, set up by his estate in 1995 to vet the thousands of prints ascribed to him, says that only the images he was directly involved in producing merit being described as his own work.

The board's decree leaves thousands of collectors, including many prominent personalities, facing the prospect that they have paid small fortunes for works that may soon be judged worthless.

The decision to redefine what constitutes a genuine Warhol stems from a revisionist view of the method he used to mass produce his famous images of celebrities and Campbell's soup tins. Many of the thousands of prints churned out by his New York studio, the Factory - and subsequently sold as Warhols - were manufactured by assistants working under his supervision. In some cases, his involvement was even more remote than this: acetates featuring original compositions travelled hundreds of miles to workshops where they were turned into prints by people he had never even met.

About 15 per cent of Warhols so far viewed by the board have had their authentication rescinded. Some critics view the board's decision as misguided at best and, at worst, a cynical ploy to inflate the value of its own stock of Warhols. The board is funded by the Andy Warhol Foundation for the Visual Arts, which oversees the ongoing sale of pictures by his estate.

Among those who have lost out because of the decision to de-authenticate works is Joe Simon, an American screenwriter who has had several images rejected. His disputed works open a philosophical Pandora's box about what should be defined as "original art". Among them is a 1965 red silkscreen self-portrait on canvas, which he bought for £115,000 in 1989. The picture had previously been authenticated by Fred Hughes, the Warhol estate executor.

Warhol purportedly authorised Richard Ekstract, a magazine publisher, to make copies of a self-portrait in exchange for a loan of video equipment, and images were produced from the original acetates by printers who were never directly employed by the artist.

Mr Simon, whose self-portrait "was seen as a Warhol for 38 years", accuses the board of rewriting history. "Nowadays, artists like Jeff Koons and Damien Hirst come up with ideas and the actual work is often physically made thousands of miles away."

In an interview with The Art Newspaper, Mr Simon also questioned the board's actions as "a perfect mechanism for removing as many Warhols from the market as possible, to preserve the scarcity and value of the multimillion-dollar stock of Warhols which was controlled by the small, tightly knit group around the foundation."

Ron Spencer, the board's lawyer, vehemently denied there was any financial motive for its actions. He said: "A work that the artist conceives, authorises, then supervises is the work of the artist. Unless all those factors are there then it is not the work of the artist."

But veteran art critic Brian Sewell said Warhol's estate was making "a false distinction".

"In the days of my misspent youth I was an occasional visitor to the Factory and I've seen his assistants saying to him things like, 'what shall we do with this, what shade shall we make this'. He would reply, 'don't know - you do it'. If he could leave to his assistants decisions on even the tone and colour of a print, I don't think the board has a leg to stand on."