A teenaged artist who was forced to stop selling his collages when Damien Hirst sent threats to his gallery (the collages incorporated ironic images of Hirst's diamond-encrusted skull sculpture) is now facing a possible jail sentence because he took a box of pencils from a Hirst installation as a prank and offered to return them only if Hirst would let him go back to displaying and selling his art. Hirst claims the box of pencils — Faber Castell Mongol 482s from 1990 — is worth GBP500,000, making this one of the gravest modern art thefts in British history.

Taking revenge, Cartrain took the box of pencils that were part of Hirst's sculpture, Pharmacy, which was being shown as part of its Classified exhibition that closed at the end of last month.

He then created a "wanted"-style poster that read: "For the safe return of Damien Hirst's pencils I would like my artworks back that DACS and Hirst took off me in November. It's not a large demand… Hirst has until the end of this month to resolve this or on 31 July the pencils will be sharpened. He has been warned."

Yesterday, Cartrain told The Independent: "I went to the Tate Britain and by chance had a golden opportunity to borrow a packet of pencils from the Pharmacy exhibit. That same day I made up a fake police appeal poster advertising that the pencils had been removed from the Tate and that if anyone had any information they should contact the police on the phone number advertised.

"A few weeks later I went out and I returned home to find out the art and antiques squad from New Scotland Yard had called round with a warrant for my arrest…"

But that is not the end of it. Police also arrested Cartrain's 49-year-old father, who they suspected of harbouring the pencils. "Initially, we arrested his dad but it soon became clear that it was his son who was responsible," said a police source. "We arranged to arrest him by appointment. The act of theft was clearly a stunt to gain publicity."