The YouTube video that claims to have caught Banksy at work

A video claiming to show graffiti artist Banksy at work around the capital while he explains his motives is becoming a major hit on YouTube.

The clip features an interview with a man who, in a thick West Country accent, claims to be the anonymous guerrilla artist, and follows him around London as he creates several works.

In keeping with the Bristol-born artist's desire to keep his identity secret, the face of the man in the film is never seen, and he wears a large hat throughout.

His agents refused to comment on the video, which shows the man whitewashing a wall to create a blank canvas before painting an image depicting the Mona Lisa using a bazooka gun, believed to have been in Noel Street, Soho.

It has since been painted over. The artist defends his work as he paints, asking: "Who is doing worse? Is it the billboard people or the writers?"

He also boasts of being able to finish a piece in under a minute.

"In London you can get a really good image up in 30 to 40 seconds. I believe what I am doing is fair enough and makes the city look better." He even claims the police have helped hone his skills.

"The Old Bill are a big part of the creative process, they are like this third party helping you make decisions on the design and the amount of detail you use."

The video also shows the artist climbing over walls to paint a railway bridge.

"If a train driver sees you they call the British Transport Police straight away," he said.

In the film he claims to be half entertainer, half activist. "If you can be arsed to get up in the middle of the night, go out armed with paint and paint pictures that people see when they are on the train on the way to work then you are in the entertainment business. You're also in the start-a-revolution business, and a bit of both is probably ideal." If the film is of the real Banksy - who has become a seriously collectable artist in recent years, selling a piece for charity at Sotheby's New York in February for almost £1million - it will be the first time his fans have been given a detailed insight into his working methods.

However, alongside the authentic Banksy pieces painted on buildings across London, there is an increasing trend for others to create "homages" to his work, complete with stencilled signatures, leading to confusion about what is, and is not, a genuine Banksy.