His debut novel exposed the drink and drug-addled underbelly of 1980s Edinburgh, recounting the gruesome exploits of junkies and psychotic alcoholics. Now, 15 years on, Irvine Welsh, the author of Trainspotting, is to write a prequel to his best-seller.

The new novella by the Edinburgh-born writer, which is expected to be published next year, will chart the decline of Trainspotting’s main characters such as Mark Renton and Simon “Sick Boy” Williamson from “daft young guys” to the debauched heroin addicts portrayed by Ewan McGregor and Jonny Lee Miller in the 1996 film adaptation.

Like Trainspotting, which was published in 1993, the new book will be set in Edinburgh’s Leith district, the centre of the 1980s Scottish heroin scene, and in London.

Welsh decided to write the prequel after stumbling upon old notes that he had written for Trainspotting in his attic.

“The thing is basically a prequel,” he said. “It’s about how Renton and Sick Boy went from being daft young guys just out for the buzz on drugs, to total junkies.

“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.”

It will be the second time that Welsh, 49, has returned to the characters and story which made his name. In 2002 he published Porno, a sequel to Trainspotting in which Renton and Williamson turn their hand to making pornographic films. The novella is unlikely to stray far from the squalor and graphic content which have become Welsh’s trademark.

Trainspotting sold more than 1m copies in the UK and spawned the Bafta-winning film directed by Danny Boyle, which became a global box office hit.

Welsh, a former heroin addict who wrote his grittily realistic first book while working in the housing department of Edinburgh council, now divides his time between his homes in Dublin and Miami.

His next novel, Crime, will be published in July and he is also compiling an anthology of short stories for publication next year.

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