Aisle be! Shopping trolley on rock fools art experts

it may have been an obvious hoax - a "cave painting"of a primitive man pushing a supermarket trolley - but it slipped past the expert eyes of the British Museum.

In his latest coup, guerrilla artist Banksy left the painting, on a 10in by 6in piece of rock from Peckham, in the museum's gallery of artefacts from Roman Britain.

It was only when he went on to write about it on his website - offering readers a prize if they photographed themselves next to it - that museum staff finally caught on.

The museum admits it has no idea how long the stone was on display, but is hoping the incident attracts new visitors.

Banksy, who has exhibited in a similar vein at Tate Britain, placed the rock painting in gallery 49, alongside a limestone statue and a 1st Century tombstone found in Tower Hill.

He placed a caption alongside it, stating: "This finely preserved example of primitive art dates from the Post-Catatonic era and is thought to depict early man venturing towards the out-of-town hunting grounds."

The museum's communications director Hannah Boulton said: "We are not treating this seriously at all.

Banksy's piece, she added "was rather well installed, but it did no damage".