Ever wondered how Sick Boy, Renton and Spud ended up in their drug-fuelled squalor, choosing heroin over life and over-sized televisions?

Fifteen years on and all is about to be revealed, as acclaimed and controversial Scottish writer Irvine Welsh sets out to write the prequel to his gritty, brutally realistic tale.

The new book, technically the third instalment after the Trainspotting sequel Porno was published in 2002, will chart the decline of the two main characters from young Scottish innocents to psychotic junkies, their lives increasingly defined by the drug culture of 1980s Edinburgh.

The decision to write a prequel came about by chance, after Irvine found some forgotten material in his attic.

'I had a great deal of material that, for various reasons, wasn't suitable for the previous books,' said Welsh in an interview with The Independent.

'I thought they'd been slung out ages ago.'

The book will be published by Random House in 2009.