Stuart, 44, has described himself as “a working-class kid who had a different career and came to writing late”. He is the second Scot to win the 50,000 pound award after James Kelman took the prize in 1994 with How Late It Was, How Late, a book Stuart said “changed his life” because it was the first time he saw “my people, my dialect, on the page”.

Shuggie Bain follows Shuggie as he attempts to care for his alcoholic mother, Agnes, whose descent into alcoholism coincides with her youngest son’s growing awareness of his sexuality. The novel is dedicated to Stuart’s mother, who died of alcoholism when he was 16.

Shuggie Bain was rejected by 30 editors before it was picked up by publishers Grove Atlantic in the US and Picador in the UK. Stuart, who was born and raised in Glasgow, moved to New York at 24 to work in fashion design after graduating from the Royal College of Art in London.

Upon learning he had won, Stuart tearfully described himself as “absolutely stunned” and thanked his mother, who is “on every page of this book – I’ve been clear without her I wouldn’t be here, my work wouldn’t be here”.

He also thanked “the people of Scotland, especially Glaswegians, whose empathy and humour and love and struggle are in every word of this book”.

Stuart, who has already written his second novel, titled Loch Awe, pointed to Kelman’s Booker winner behind him on his shelves. “When James won in the mid-90s, Scottish voices were seen as disruptive and outside the norm. And now to see Shuggie at the centre of it, I can’t express it,” he said. “Young boys like me growing up in 80s Glasgow, this wasn’t ever anything I would have dreamed of.”

He said he would now become a full-time writer, and joked that his winnings would be spent on settling his bet with his husband that he wouldn’t win. More seriously, he said he might use the money to return to Glasgow.

Margaret Busby, a publisher and the chair of this year’s Booker judges, said the work was “destined to be a classic”, describing it as “a moving, immersive and nuanced portrait of a tight-knit social world, its people and its values”.

“It is such an amazingly emotive, nuanced book that is hard to forget. It’s intimate, it’s challenging, it’s compassionate,” she said, describing Shuggie as “an unforgettable character”.

Stuart was one of four debuts among the six novelists to be shortlisted for this year’s Booker prize, whittled down from 162 novels. The final six contenders made up the most diverse lineup in the prize’s history, with Stuart beating the US writers Diane Cook, Avni Doshi and Brandon Taylor, the acclaimed Zimbabwean Tsitsi Dangarembga and the Ethiopian-American Maaza Mengiste.

After last year’s judges provoked controversy by flouting the rules to choose two winners, Margaret Atwood and Bernardine Evaristo, this year’s judges’ meeting was “unanimous and quick”, said the Booker’s literary director, Gaby Wood. She added that “guidelines” had been added to the prize so that if the judges were split again, the majority vote would be honoured.

Normally announced at a formal dinner in London’s Guildhall, this year’s prize was announced in a BBC broadcast from the Roundhouse in Chalk Farm, north London, with the shortlisted authors joining in from their homes around the world.

The ceremony was moved forward by two days, ostensibly to avoid a clash with former US president Barack Obama’s memoir, A Promised Land; last week, it was announced that Obama would be taking part in the Booker ceremony. In a pre-recorded message, Obama offered “my congratulations and admiration” to the nominated authors, citing Marilynne Robinson, Colson Whitehead and Bernardine Evaristo as past Booker-nominated authors who had offered him “a brief respite from the daily challenges of the presidency”.

The Booker has been criticised for having opened up entries to any author writing in English in 2014, with the British literary scene fearful the rule change would lead to dominance by Americans. This year, apart from Dangarembga, all the shortlisted writers were from the US or held joint US citizenship.