Speculation about the artist's identity was rife after the work, on a disused river crossing, was spotted by passers-by on Thursday.
The stencilled design depicts a child carrying a wooden sword with a pencil attached to the end, under the text "Draw the raised bridge!"
Images of the mural were shared on Banksy's official Instagram account on Friday.
The elusive street artist posted two photos of the permanently raised bridge, situated on Scott Street in the Wincolmlee area of the city. Source: BBC