Jack Cowart, director of the Roy Lichtenstein Foundation and head of the Council of Artist Foundations, says there is no standard way that foundations address issues of authenticity. Some rely on experts to pass judgement while others depend on catalogue raisonnes. Some send detailed letters of explanation, others do not. But, the Warhol Foundation has "the most multi-layered, lawyered-up and persuasive process I know," he says. The auction houses rely on the board to determine which consignments to include in their sales. "We pass things by them and they have the final word," says a spokesperson for Christie's.

Some dealers and collectors see a conflict in the Authentication Board receiving funds from the foundation, which itself sells Warhols from the estate. Board lawyer Ron Spencer rejects the implication of collusion, noting that the Authentication Board is separately incorporated and "entirely independent of outside influences, including the Warhol Foundation itself which has absolutely no input or influence on the board's decisions."

The board's current members-Warhol experts Neil Printz and Sally King-Nero, art historian Robert Rosenblum, and curator David Whitney-are "independent scholars who have their own reputations to protect," says Mr Spencer.

But Mr Simon sees them as "a perfect mechanism for removing as many Warhols from the market as possible, to preserve the scarcity and value of the multi-million dollar stock of Warhols which is controlled by the small, tightly knit group around the foundation."

The Authentication Board secretary, Claudia Defendi, reports that over a two-year period "the percentage of works denied authentication is roughly 15%." The Warhol Foundation is also funding the project to publish a catalogue raisonne of Warhol's works, a collaborative project with Zurich dealer Thomas Ammann Fine Arts. Since Mr Ammann's death in 1993 his sister Doris Ammann has carried on the research with foundation support. Last year Phaidon published the first of the five volumes.

Authentication Board members are also working on the catalogue raisonne-Mr Printz is co-editor, Ms King-Nero is executive editor, and Mr Rosenblum serves as a consultant. The foundation has sold works to Ammann Fine Arts, however, no member of the board works for the Swiss gallery or any other commercial entity. The board does not explain its decisions because to do so would provide "a roadmap for forgers", says Mr Spencer, who says the letter to Mr Simon was an "exception" and does not herald a policy change. Until the board makes known the basis for its opinions, particularly concerning works produced by assistants, the controversy will only become more intense. An article by Michael Schnayerson in Vanity Fair's November issue will add fuel to the fire, with a wealth of anecdotal evidence from disgruntled owners, including Mr Simon and Mr Ekstract, who has had his own work rejected by the board.

Mr Simon would already be in court were it not for the document he signed waiving the right to sue when he submitted his works for authentication. That compulsory waiver has not stopped everyone. New York dealer Ivan Karp sued the board but dropped his claim when told why the board had rejected his work.

"The knee-jerk reaction is litigation," says Mr Cowart of the Lichtenstein Foundation, "but if you have a definitive archive and a clear objective process, usually the attribution will be sustained." Mr Simon alludes to a class-action suit being brought against the board, but Mr Spencer is confident that, if it comes to that, the courts will uphold the Authentication Board 's decision. Jason Edward Kaufman.

(note: The attorney for the Lichtenstein Foundation happens to be the same as for the Warhol Board, Ron Spencer; George Frei of Thomas Ammann Fine Art was an original member of the Authentication Board).