He claimed the work on Instagram.
In the video, which begins like Bob Ross' The Joy of Painting, we see Banksy using a giant stencil to make the painting.
The choice of Reading prison is not insignificant for this provocative reason.
Oscar Wilde was imprisoned there for "serious immorality" when he criticized the British prison system at the time.
The prison has not housed any prisoners since 2013.
The Ministry of Justice will soon decide whether or not to transform it into a place of entertainment and culture.