The 14-foot painting, which depicts bickering chimpanzees debating in the British Parliament, will hit the auction block on October 3 in London.
Sotheby's announced earlier this week that the piece could sell for between $1.5 million and $2.0 million, potentially setting a new auction record for the anonymous street artist.
However, Banksy aficionados have noticed several inconsistencies between the work being auctioned off as "Devolved Parliament," and the painting shown for the first time in 2009 at the Bristol Museum under the title "Question Time."
As the New York Times pointed out, the work displayed in the "Banksy vs. Bristol Museum" exhibit featured two large chandeliers illuminating the House of Commons. The lamps have disappeared in the version for sale by Sotheby's, giving a darker tonality to the painting.
In the left foreground of the 2009 version, a seated chimpanzee is holding an upturned banana, while the fruit is inexplicably facing downward in "Devolved Parliament."
In addition, a piece of decorative carving of the parliamentary benches has changed shape and size in the two versions.
When reached for comment about the differences, Sotheby's confirmed to the New York Times that "Devolved Parliament" is the same work as the one shown in Bristol a decade ago.
The original catalog description mentioned that the painting was signed and dated "2009," and that the current owner acquired it from Banksy in 2011. It presented the work under the new title "Devolved Parliament," without mentioning the alterations.
A representative for Sotheby's told the New York Times that the painting has been modified by Banksy himself since it was first exhibited at the Bristol Museum, without sharing additional details about the timeline for the alterations.
The auction house also assured that "Devolved Parliament" is unique and that there is no other versions of the work.
The painting was exhibited again at the Bristol Museum earlier this year to mark the 10th anniversary of the 2009 exhibition "Banksy vs. Bristol Museum," as well as the so-called "Brexit Day" on March 29.
At the time, the art institution made no mention of "Devolved Parliament" being different from the work shown in 2009.