Ten bidders were in the contest for Devolved Parliament to take it well above the top estimate of £2m and the artist’s previous auction high of $1.7m.
Banksy first showed the painting, then titled Question Time, a decade ago as part of the Banksy vs. Bristol Museum exhibition in 2009.
The painting has since been reworked by the artist and retitled. Once glowing, the Commons’ lamps have been dimmed, while an upturned banana in the foreground now faces downwards.
It was shown again in Bristol on March 29 with the artist posting on Instagram: “I made this 10 years ago. Bristol museum have just put it back on display to mark Brexit day. Laugh now, but one day no-one will be in charge.”
At close to 13ft (4.46m) across, this is the largest known oil on canvas by the street artist whose auction reputation has leapt since last year’s stunt to shred a picture shortly after its sale at Sotheby’s.
The previous record for Banksy was the $1.7m bid at Sotheby’s New York in 2008 for Keep it Spotless, a Damien Hirst spot painting defaced to include a Los Angeles hotel maid.