Damien Hirst in vicious feud with teenage artist over a box of pencils

How much is a box of pencils worth? Fifty pence? £3.99 if the pencils have rubbers on the ends? Well, if they're part of a Damien Hirst art installation, the value is £500,000. That is what 17-year-old graffiti artist Cartrain discovered when he pilfered some pencils from Hirst's sculpture Pharmacy. And that wasn't all – he was arrested, released on bail, and is waiting to find out if he will be formally charged with causing damage to an iconic artwork worth £10m.

When Cartrain walked into Tate Britain and made off with a few HBs in July, he believed it was a harmless game of tit-for-tat as part of an ongoing feud. He originally locked horns with the millionaire artist last year, when he used an image of Hirst's famous diamond-encrusted skull, For the Love of God, to create collages that were put up for sale on an art website.

Hirst reported him to the Design and Artists Copyright Society and a string of legal letters were sent to Cartrain's art dealer, Tom Cuthbert, at 100artworks.com, about the teenager's pieces, also called For the Love of God. The online gallery surrendered them to Hirst with a verbal apology.

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."

He was told by custody officers that the pencils were valued at £500,000 and that he had damaged "the concept of a public artwork titled Pharmacy ... valued at £10,000,000". Cartrain is on bail and, if convicted, his actions will feature among the highest value modern art thefts in Britain. The box of pencils – a very rare "Faber Castell dated 1990 Mongol 482 Series" – will be put back by Hirst, although the installation is no longer on public display.

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."

A statement from Tate Gallery confirmed: "On Saturday 4 July 2009, a member of the public removed a box of pencils from the desk in Damien Hirst's installation Pharmacy. The matter is being investigated by the police."