When Trainspotting exploded on to the literary landscape 15 years ago, the impact was as potent as the heroin shooting through the veins of its characters. But for fans who wondered how the characters Renton and Sick Boy came to choose drugs over life, jobs and oversized televisions in the squalor of 1980s Edinburgh, all is about to be revealed.

Irvine Welsh is planning to write a prequel to the cult novel. The novella will chart the decline of the original book’s colourful rolecall of characters, from young innocents to the psychotic band of addicts and alcoholics whose fictional exploits came to define an era. “The thing is basically a prequel and will be about how Renton and Sick Boy went from being daft young guys just out for the buzz to total junkies,” Welsh said.

The novella, which is expected to be published next year, will technically be the third instalment of the story. In 2002 Welsh, 49, brought out a sequel, Porno, in which the two characters, Mark Renton and Simon “Sick Boy” Williamson, had moved into the adult movie business.

It will focus on the journey of the two main characters, introduced in the film version by the young and at the time unknown actors Ewan McGregor and Jonny Lee Miller, as they become immersed in the seedy subculture of the Scottish capital at the end of the 1970s. Welsh, who now divides his time between homes in Dublin and Miami, said: “It focuses on them when they are a couple of years younger and shows how their attitudes and behaviour start to change as they become more defined by the drug and the culture around it. I had a great deal of material that, for various reasons, wasn’t suitable for the previous books.”

The Edinburgh-born writer said that he decided to publish the backstories after finding what he described as “lost” notes in his attic. Renton and Sick Boy’s first visit to London, cut from Trainspotting because it was too long, is likely to be included, he said, along with other ideas sketched out for the original book.

Trainspotting sold more than a million copies in Britain before being turned into a Bafta-winning film. The film helped to launch the careers of Scots actors McGregor, 36, Miller, 35, and Robert Carlyle, 46, who starred as the psychopath Francis Begbie.

As McGregor has already ruled out appearing in a film version of Porno, it is unlikely that the actors could be persuaded to team up for an adaptation of the new, unnamed novella.

Ian Rankin, creator of the more respectable Edinburgh character Inspector Rebus, said that the backstory could have considerable artistic potential. “You don’t find out where the characters come from, so there is potential there for that to be explored,” he toldThe Times. “There is a very good commercial reason for [the new novella], which is that Irvine Welsh is best known forTrainspotting, and obviously the characters meant a lot to him.”

Aaron Kelly, a lecturer in English at the University of Edinburgh, predicted that there would be a huge demand for the prequel. He said: “Irvine Welsh wasn’t the first to use phonetic dialect, but there was nobody else doing it at the time and it was one of the real strengths of Trainspotting.”

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