In a statement released by his publicist the day before L&T's sale of modern and contemporary art and design in Marylebone on September 27, Banksy refused to authenticate five street works at the sale estimated at over £200,000.
These works had been removed from their original settings and had been approved by Vermin, the recently-established authentication service made up of a "board of experts" set up as an alternative to the Banksy-approved verification panel, Pest Control.
All five works failed to sell, with the major casualty being Fungle Junk - three panels from a larger mural that Banksy purportedly sprayed on the side of a trailer at the Lizard Festival in Cornwall in 1999. It was unsold against a £100,000-150,000 estimate having been given full Vermin authentication.
"We are sure these street works are authentic," L&T specialist Ben Hanly told the London Evening Standard, which published a story immediately after Banksy's statement was released.
"It would be absolute madness for us to sell anything we don't believe is genuine," he added.
Banksy's statement was clearly timed to inflict maximum damage on the sale and undermine the status of Vermin as an accreditation source for his work.
In all, only five of the 24 works by Banksy at the sale managed to find buyers and overall the selling rate for the 283 lots was under 30 per cent - indicating the difficulties in shifting lower and middle range contemporary art at auction generally.
Banksy's statement read: "For the sake of keeping all street art where it belongs, I'd encourage people not to buy anything by anybody unless it was created for sale in the first place."
A further statement posted on the Pest Control website stated: "All works authenticated by Pest Control have been done so in conjunction with the artist. Banksy does not provide this service through any other third parties and we would caution collectors against relying on such bodies."
The reasons behind Banksy's protective stance are thought to be threefold. Firstly, public acknowledgement of original street works may open the artist up to potential prosecution for vandalism. T
hen there is the issue of fakes - Pest Control say they have identified 89 street pieces and 137 screen prints falsely attributed to the artist so far this year.
Thirdly, Pest Control, which is also closely associated with Banksy's primary dealer Lazarides, wish to maintain their position as the sole authentication body and thereby control the supply of approved works on the market.
While L&T stated in the catalogue that they consider "the certification service provided by Vermin will prove to be a highly useful tool" in the authentification process, Sotheby's and Dreweatts are among a number of auctioneers that will only offer Banksy's works with a Pest Control certificate.
Indeed, the top Banksy at L&T's sale was one of 50 screenprints of Bomb Middle England that did have a Pest Control certificate, although it sold below estimate at £13,000.
Where the results of this sale now leaves Vermin is unclear, and the auction certainly does little to help L&T's attempt to establish themselves as a force in the London contemporary scene.
It now remains to be seen if there will be any knock-on effect on the Urban art market. The L&T results coincide with a difficult moment in the financial industry, and investors and hedge fund managers are known to have taken a particular interest in this field.
The next test will come at Dreweatts' urban art sale in Shoreditch on October 14 which includes 11 Banksys, all with Pest Control certificates.
Dreweatts chairman Stephan Ludwig told ATG: "Buyers of Banksy's original works are acquiring, to a degree, a connection with the artist, which is of course reinforced by authentication. Pest Control, as his de facto office, stands alone in being able to properly provide this service. Dreweatts will not offer unauthenticated urban art for sale."
A further report of the L&T sale appears in the print edition of this week's ATG.