Using the GPS in their mobile phones to track their movements, gamers connect with each other trying to buy drugs including cannabis, cocaine and ecstasy at low prices.

They then move around cities such as London, Manchester and Birmingham trying to sell them on for a profit, making trades with real people online.

Makers say of the game Underworld (initially called DrugLords) : "You become a drug dealer in the criminal underworld, which exists parallel to your everyday life."

The game is expected to be available from the Apple iTunes store just in time to outrage anti-drugs charities for Christmas.