Graffiti artist Banksy has been caught up in a scam involving online auction site eBay.
Banksy's controversial portrait of Winston Churchill
According to The Art newspaper, unauthorised prints were sold at artificially inflated prices, with false bidders driving the price up.
Banksy's dealer, Pictures On Walls, said in spite of "strict controls", 25 "bad prints have been sold on eBay".
An investigation is being launched and concerned Banksy owners are being asked to come forward.
In a statement to the Guardian newspaper, Pictures On Walls said: "Pictures on Walls have called on eBay to assist in tracing these sales and also in tracing the money which will inexorably lead to those that have cynically betrayed the trust of the public, the artist and the company."
eBay told the BBC News website that maintaining a secure site for buyers and sellers was its "first priority".
"We invest millions of pounds each year in securing the safety and security of the site; indeed, we employ over 2,000 people whose sole responsibility is to this area," it said.
Blur album cover
"Following our own investigations, we can confirm that four users have suspended from our site."
Prices for Banksy's works have rocketed in the past year as a result of his frequent appearances in the media and the patronage of celebrities including Angelina Jolie.
His painting Space Girl and Bird, commissioned for a Blur album cover, made a record £288,000 at auction in April.
Public interest has also been piqued about who exactly the Bristol-born artist is as he tries to keep much about his identity secret.
Warning against fakes
Charles Dupplin, art expert at specialist art insurer Hiscox, warned that buyers must always be "careful about the authenticity and the provenance of any piece they are buying".
"Crooks are in the art market as anywhere else," he said.
He added that the art world was seeing a "huge level of interest in prints by contemporary artists such as Banksy and Anthony Micallef", and that they were being bought by younger collectors.
"The internet is a key channel for these purchases but this can mean that buyers are vulnerable to fakes," he said. "If there is any doubt in a buyer's mind we advise them to go through reputable dealers."