The writer is to reunite Renton, Begbie, Sick Boy and Spud for a new adventure in his novel Dead Men’s Trousers, which will be published in March.
Edinburgh-born Welsh revealed it will feature the friends in their fifties and will complete the story as he didn’t want to write about them as pensioners.
A plot summary released by publishers Penguin last year revealed that one of the much-loved characters will not survive until the end of the book.
Welsh, 59, has already written four best-selling novels featuring the characters; Trainspotting, Porno, Skagboys and The Blade Artist.
In an interview with The Bookseller ahead of publication in March, he said: “This has to be the final novel really, unless I write about some drama taking place in an old folks home.
“I can’t really see these characters existing in a dynamic way anymore. I see the social and dramatic vitality of these characters as definitely being gone after this.”
He added: “It carries on where the last book, The Blade Artist, ended. Begbie and Renton are on a plane. It’s about the characters from Trainspotting and where they are now.
“They’re guys in their fifties and they are at a funny time in life when you’re plodding on with your interests but also more driven to looking back on things as well.
“So everybody’s in a state of anxiety, and these characters are in this state as well.”
The new book is set in Edinburgh against the backdrop of Welsh’s football team Hibs’ Scottish Cup Final victory over Rangers in May 2016.
Renton, played by Ewan McGregor in the films, is now a successful manager of international DJs but is bored with his nomadic existence of constant travelling.
He bumps into old rival Begbie, portrayed by Robert Carlyle in the film version, who has mellowed after becoming an artist and doesn’t want to take revenge on him.
Meanwhile, Sick Boy and Spud, played by Jonny Lee Miller and Ewen Bremner respectively, are desperate to make cash and end up involved in the grim practice of organ harvesting.
Welsh has said the story could form the basis for a third Trainspotting film following the success of T2 Trainspotting, which was directed by Oscar-winner Danny Boyle.
The blurb for the book released by Penguin reads: “Mark Renton is finally a success. An international jet-setter, he now makes significant money managing DJs, but the constant travel, airport lounges, soulless hotel rooms and broken relationships have left him dissatisfied with his life.
“He’s then rocked by a chance encounter with Frank Begbie, from whom he’d been hiding for years after a terrible betrayal and the resulting debt.
“But the psychotic Begbie appears to have reinvented himself as a celebrated artist and – much to Mark’s astonishment – doesn’t seem interested in revenge.
“Sick Boy and Spud, who have agendas of their own, are intrigued to learn that their old friends are back in town, but when they enter the bleak world of organ-harvesting, things start to go so badly wrong.
“Lurching from crisis to crisis, the four men circle each other, driven by their personal histories and addictions.”