More than 20 art dealers and collectors are preparing to sue the board that oversees Andy Warhol's $700 million (£413 million) legacy over allegations that it is deliberately refusing to verify genuine works by the artist. The collectors claim that the Andy Warhol Authentication Board, which is responsible for deciding on the provenance of works of art by Warhol, is declining to acknowledge new discoveries because it wants to ensure that the price of existing works remains high.

Many of those whose claims have been turned down by the board claim that they were given the works by Warhol himself when he was still alive. They are now seeking a change in the law governing how mass-produced works such as Warhol's are valued. Warhol often gave his technicians, cleaning ladies and security staff pictures to act as their "pension" - many of which they now find to be worthless.

The campaign against the authentication board is being led by Joe Simon, the producer of the film, Richard the Third. Mr Simon, who lives in London, has had a number of works turned down, including a Warhol silkscreen self-portrait that he bought 14 years ago for $195,000. The work, he said, had been previously sold by Christie's and verified by Warhol's manager, Paul Morrissey, and by Fred Hughes, Warhol's executor and the late chairman of the Andy Warhol Foundation.

Mr Simon himself bought it from the established gallery Lang & O'Hara in New York. In addition, both the Warhol estate and foundation had verified the work as genuine in 1987 and 1988. When Mr Simon presented it to the board again three years ago, however, with the intention of selling it for £2 million to a buyer who wanted proof it was genuine, he was told that it was "not the work of Andy Warhol".

"I felt very angry - largely because they had encouraged me to submit it so the picture might be included in the catalogue that the board are involved in editing," he said. "To my utter disbelief, my twice previously officially authenticated painting was returned to me denied, without any explanation. To this date they still refuse to give any reason." Those who feel equally angry include Billy Name, Warhol's photographer, who worked with the artist and lived in his "factory" between 1964 and 1970. Mr Name has put forward 157 Polaroid photographs taken for Esquire magazine in 1968, all of which he says were given to him by Warhol but which have been turned down by the board as not genuine.

Richard Ekstract, the successful publisher to whom Warhol turned for financing of the magazine Interview, has also submitted a Warhol self-portrait screenprint, which has been refused. Mr Simon said: "Every day I get another email from someone climbing on board. It's nice to know I'm not alone. I have realised that there are many others with genuine cases suffering at the hands of the board." The serious problem facing the authentication board, however, is that Warhol, having come up with the concept for a work, often delegated the manual labour to other people - thereby making it difficult to ascertain who "made" the piece.

John Paul Russell, his printer during the 1980s, said: "I had never seen Andy Warhol even once come down to the studio in Tribeca to watch his work being printed." Many of the artist's instructions to the printers were by telephone. To further complicate matters, Warhol would often pay printers with proofs that were not registered as part of the official edition.

Ron Spencer, the lawyer for the authentication board, said that its role was to ascertain the "intent" of the artist. He told this month's Vanity Fair magazine that even if a work were printed by a number of other people the board would still classify it as genuine. "If Warhol conceived the idea and he then directed someone else to prepare a silkscreen, supervised the process of production and, in effect, signed off on it, as long as he said, 'That's good, that's what I wanted', Warhol created that work."

Many dealers claim, however, that this definition does not hold in practice and that some screenprints in a series are accepted as genuine by the board while identical works are refused. Warhol's paintings are now among the most expensive pieces of 20th-century art. One of his screenprints of a Campbell's soup tin costs £10 million and one of Marilyn Monroe £11 million. Even a Polaroid photograph by the artist can cost up to £12,000. He died in 1987 after a gall bladder operation.

The board is compiling a catalogue of all Warhol's works. Owners of pictures still not verified fear that if their works are not included in the catalogue they will find it much harder to prove they are genuine. Paul Morrissey, Warhol's former manager, said: "The whole point about Warhol was that he mass-produced works. That's why he called it a factory."