Guerrilla artist Banksy has refused to authenticate five works up for auction in London this weekend.
A group called Pest Control, endorsed by Banksy, was set up earlier this year to authenticate the artist's canvasses and prints after a spate of fakes.
But Pest Control said Banksy does not like his art being removed from its original setting and will not approve any street pieces.
Auctioneer Lyon and Turnbull said it had no doubt the pieces were genuine.
A spokesman for the auctioneers said: "Banksy hasn't said they are fake. I don't know why he's not authenticated them... He's saying that street art should stay on the streets."
The firm's Ben Hanly said it would go ahead with Saturday's sale without official verification.

'Extreme caution'
He added that the pieces had been authenticated by an unauthorised organisation, Vermin.
On its website, Pest Control said that since its creation in January, 89 street pieces and 137 screen prints attributed to Banksy had turned out to be fake.
"Pest Control does not authenticate street pieces because Banksy prefers street work to remain in situ and building owners tend to become irate when their doors go missing because of a stencil," Pest Control said.
"He would encourage anyone wanting to purchase one of his images to do so with extreme caution, but does point out that many copies are superior in quality to the originals."
Mr Hanly insisted the five works up for sale - worth between £200,000 and £275,000 in total - were genuine.
They include Refuse Rat, which is expected to fetch £20,000, and Fungle Junk £150,000, consisting of three panels painted on to the side of a trailer at a festival in Cornwall in 1999.
"They are Banksy's. They are in all the literature and everything. And Vermin's authentication service gives all the provenance for each piece listed as clear as day," he said.
"The market will decide."