Galgut had been British bookmakers' runaway favourite to win the 50,000-pound ($A91,000) prize with his story of a troubled Afrikaner family and its broken promise to a black employee - a tale that reflects bigger themes in South Africa's transition from apartheid.

Galgut took the prize on his third time as a finalist, for a book the judges called a "tour de force".

He was previously shortlisted for The Good Doctor in 2003 and In A Strange Room in 2010, but lost both times.

Despite his status as favourite, Galgut said he was "stunned" to win.

Galgut said he was accepting the prize "on behalf of all the stories told and untold, the writers heard and unheard, from the remarkable continent that I'm part of".

He noted that this year's Nobel literature laureate, Zanzibar-born writer Abdulrazak Gurnah, was also African.

"Please keep listening to us - more to come," Galgut added.

His novel paints a troubling picture of modern-day South Africa, though Galgut said he did not set out to be negative.

"I didn't plan for the overall trajectory of the book to be a downward one," he said - though that's how it turned out.

"I think the portrait it paints of modern South Africa is not a happy one," he said.

"I had no agenda in describing it that way, but things are not great with us right now. You could read that as a warning or a portrait, I don't know, but South Africa has seen better days."

Historian Maya Jasanoff, who chaired the judging panel, said The Promise was a profound, forceful and succinct book that "combines an extraordinary story, rich themes - the history of the last 40 years in South Africa - in an incredibly well-wrought package".

Galgut's ninth novel traces members of the Swart family - the word is Afrikaans for black - haunted by an unkept promise to give their black maid, Salome, her own house.

He is the third South African novelist to win the Booker Prize, after Nadine Gordimer in 1974 and J.M. Coetzee, who won twice, in 1983 and 1999.

The Promise was selected over five other novels, including three by US writers: Richard Powers' "Bewilderment," the story of an astrobiologist trying to care for his neurodivergent son; Patricia Lockwood's social media-steeped novel "No One is Talking About This" and Maggie Shipstead's aviator saga "Great Circle."

The other finalists were Sri Lankan author Anuk Arudpragasam's aftermath-of-war story "A Passage North" and British/Somali writer Nadifa Mohamed's "The Fortune Men," about a Somali man falsely accused of murder in 1950s Wales.

Jasanoff said many of the shortlisted novels, including Galgut's, reflected on the relationship between past and present.

"This is a book that's very much about inheritance and legacy," she said of the winner. "It's about change over a period of decades. And I think it's a book that invites reflection over the decades and invites and repays rereading."

Founded in 1969, the Booker Prize has a reputation for transforming writers' careers and was originally open to British, Irish and Commonwealth writers. Eligibility was expanded in 2014 to all novels in English published in the UK

The judging panel winnowed their list from 158 novels submitted by publishers. Only one British writer, Mohamed, made the final six, a fact has renewed debate in the UK about whether the prise is becoming US-dominated.

Last year there also was only one British writer on a US-dominated list of finalists, Scotland's Douglas Stuart. He won the prize for "Shuggie Bain," a gritty and lyrical novel about a boy coming of age in hardscrabble 1980s Glasgow.

For a second year, the coronavirus pandemic has scuttled the prize's usual black-tie dinner ceremony at London's medieval Guildhall. The winner was announced in a ceremony broadcast live on BBC radio and television.

Australian Associated Press