One offer of �20,000 has already been made

Martin Trent, 42, said the spray-painted image of a monkey riding a bomb had been authenticated by a friend who knew the elusive artist.

The 2 sq ft mural, complete with a Banksy signature, was painted on the wall in Easton about seven years ago, but is now in a secure lock-up.

The work has now gone on sale, and has already attracted an offer of �20,000.

"We finished painting the wall and the next day [the mural] arrived. We could've painted over it," said Mr Trent.

"I liked it on the wall, but the other graffiti was getting closer and closer and I'd heard of a few being painted over.

"It has been pleasurable to have it over the last year, but I've realised there is a lot of interest there," he added.