LONDON — 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 honor 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 organizers then changed the rules to only allow one winner to avoid undermining either book.