Margaret Atwood and Bernardine Evaristo have both won this year’s Booker Prize, it was announced at a ceremony on Monday, after the judges for the literary award rebelled against its rules.

“We were told quite firmly that the rules state you can only have one winner,” Peter Florence, the chairman of the Booker judges, said at a news conference. But the “consensus was to flout the rules and divide this year’s prize to celebrate two winners.”

Evaristo, who won for her novel Girl, Woman, Other, is the first black woman to win the Booker Prize. “I hope that honour doesn’t last too long,” she said in her acceptance speech. Atwood, who won in 2000 for The Blind Assassin, was considered a front-runner this year for The Testaments, the sequel to her 1985 dystopian classic, The Handmaid’s Tale.

It is not the first time the award has been shared. In 1992, Michael Ondaatje’s The English Patient shared it with Barry Unsworth’s Sacred Hunger, but the prize’s organisers then changed the rules to only allow one winner to avoid undermining either book.

Several judging panels had tried to split the prize since, said Gaby Wood, the Booker Prize Foundation’s literary director, but settled on single winners after being told they had to.

The decision to rebel and award the prize to two writers this year was not taken lightly. The judges, who included the author Xiaolu Guo and editor Liz Calder, spent over three hours trying to pick a winner before asking if they could choose both. They were told they couldn’t. The judges then — to the Booker organisers’ “horror”, Florence said — spent another “hour-and-a-half agonising how to resolve the issue”, before deciding it was the only result they wanted.

They were again told it was unacceptable. It was only at a third attempt, 30 minutes later, that the Booker Prize’s trustees accepted the decision.

Wood dodged the question when asked if she supported the final result. “I support the mechanism by which the judges made their decision,” she said. She then joked the judges would not be paid for their involvement, which included reading 151 books.

For Atwood, the prize comes at a moment of renewed cultural relevance for The Handmaid’s Tale, which has sold more than 8 million copies worldwide in English. The novel was adapted into a hit television series on Hulu, and the story has taken on fresh political resonance, as women dressed as handmaids have flooded Congress and state capitols to protest new restrictions on reproductive rights.

In The Testaments, Atwood weaves together the stories of three female narrators in Gilead, a religious autocracy in what was formerly the US.

Evaristo, an experimental writer who is well established in Britain but not widely known internationally, is a more surprising choice.

In her eight works of fiction, Evaristo, who was born in London in 1959 to a white English mother and a Nigerian father, often explores the lives of members of the African diaspora. Girl, Woman, Other features a dozen characters, most of them black British women. It’s written in a blend of poetry and prose, a hybrid that Evaristo calls “fusion fiction”.

In an interview with The Times on Monday night, Evaristo said that the novel grew out of her frustration over the lack of representation in British literature.

“When I started the book six years ago, I was so fed up with black British women being absent from British literature,” she said.