Irvine Welsh to revive Sick Boy and Renton in 'Trainspotting' prequel

How did they get like that, some of literature's most drug-fuelled characters – Renton, Sick Boy, Spud and the rest of their gang in Irvine Welsh's Trainspotting? We're about to find out, because 15 years after his celebrated novel, the Scottish author intends to write a prequel.

The ultra-realistic chronicler of heroin addiction in mid-1980s Edinburgh is to go back a couple of years in his new book, to chart the decline of the Trainspotting crew from young guys looking for a good time, to the full-on smackheads so memorably brought to life in the novel (and even more in the 1996 film where Ewan McGregor's portrayal of Mark Renton launched his movie career).

Welsh has been prompted to write their beginnings by finding in his attic earlier material for Trainspotting, gathered but not used – much of it from his own years on Edinburgh's housing schemes. "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," he 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."

Besides six other novels, Welsh has already written a Trainspotting sequel, Porno (2002), where both Renton and Sick Boy are shown turning their hands to making adult movies. He says that the decision to portray them earlier as well as later came about because of a chance find.

"I had a great deal of material that, for various reasons, namely pace and because it didn't fit the time frame, wasn't suitable for the previous books," he said. "There's a particular section about Renton and Sick Boy's first visit to London to stay with their friend Nicksy in Hackney that I always wanted to publish, but it was just a bit too long.

"The others are first and second drafts from 1991 based on the same diaries and notes as the original Trainspotting. I only found them as I've been looking through boxes that have been in the attic for years – and I thought they'd been slung out ages ago."

The book became a major literary success after its original publication in 1993. The grittily, not to say unflinchingly realistic, portrayal of heroin addiction as a part of Scottish working-class culture, with casual and extreme violence often a characteristic of degraded lives, repelled and even outraged some people but also drew enormous critical admiration. A particular feature of it was that the speech of the main characters was recorded in Scottish dialect, written down phonetically.

After selling more than one million copies in the UK, the novel spawned the Bafta-winning, Oscar-nominated film of the same name directed by Danny Boyle, which became a worldwide success. Besides launching McGregor's career, the firm was a launchpad for Jonny Lee Miller who played Simon "Sick Boy" Williamson, Ewen Bremner, who played Spud, and especially for Robert Carlyle, who played the violent psychopath Begbie.

Dr Aaron Kelly, a lecturer in English at Edinburgh University and an expert on the author, said there would be a huge demand for the prequel. He said: "It sounds like a great idea to me."

Himself a former addict, who wrote Trainspotting while working in Edinburgh council's housing department, Welsh now divides his time between homes in Miami and Dublin. Last year, he published the short story collection, If You Liked School, You'll Love Work, and the novel, The Bedroom Secrets of the Master Chefs. His next novel, Crime, will be published in July and he is also compiling an anthology of short stories for publication next year.