The British Museum said the rock was "in keeping with the other exhibits"

The rock was put there by art prankster Banksy, who has previously put works in galleries in London and New York.

A British Museum spokeswoman said they were "seeing the lighter side of it". She said it went unnoticed for one or two days but Banksy said three days.

Banksy also hung a sign saying the cave art showed "early man venturing towards the out-of-town hunting grounds".

It read: "This finely preserved example of primitive art dates from the Post-Catatonic era.

It looked very much in keeping with the other exhibits, the explanatory text was quite similar

British Museum spokeswoman

"Most art of this type has unfortunately not survived. The majority is destroyed by zealous municipal officials who fail to recognise the artistic merit and historical value of daubing on walls."

Banksy is best-known as a graffiti artist who has attracted a cult following for stencilled designs that satirise authority and modern society.

He hung his own art in the Tate Britain in London in October 2003, which was not noticed until it fell to the ground, and has done the same in four New York galleries.

Banksy has previously stuck a painting to the wall of Tate Britain

A spokesperson for Banksy said he sneaked the work into the museum on Monday and it was found on Wednesday.

He ran a competition on his website for fans to have their photographs taken with the rock, offering a shopping trolley as a prize.

A British Museum spokeswoman said: "We're reasonably confident that it hadn't been up for that long, maybe a couple of days.

We have loaned the rock to Banksy but we are still in the process of deciding what to do with it

British Museum spokeswoman

It is now being exhibited at Banksy's new show, Outside Institute, which opens in London on Friday. It will have a sign saying it is "on loan from the British Museum".

The British Museum spokesperson said they were expecting it back when Outside Institute ends in June.

"He has said to us that we can keep it," she said.