Judges tore up the rule book on Monday, awarding the prestigious Booker Prize for Fiction jointly to Canadian author Margaret Atwood for "The Testaments" and Anglo-Nigerian author Bernardine Evaristo for "Girl, Woman, Other.

Atwood becomes only the second female author to win the award twice, sharing the £50,000 ($62,800, 60,000 euros) prize at the 50th anniversary ceremony at London's Guildhall.

The award has been shared twice before, in 1974 and 1997, when the rules were changed to supposedly prevent it from happening again.

The 79-year-old Atwood, who wore a badge of climate activist group Extinction Rebellion, held aloft the arm of her fellow winner as they walked to the podium together.

"I'm very surprised, I would have thought I would have been too elderly," said Atwood, who was honored for her best-selling sequel to her 1985 dystopian classic "The Handmaid's Tale".

Margaret Atwood poses with Bernardine Evaristo. Reuters

"I don't need the attention, so I'm very glad that you're getting some," she said to Evaristo, joking that as "a good Canadian, we don't do famous, we think it is in bad taste, so it would have been embarrassing if I'd been alone here."

Evaristo responded that it was "so incredible to share this with Margaret Atwood, who is such a legend.

"I am the first black woman to win this prize," she added, to cheers from the audience.

Canadian author Margaret Atwood poses with her book 'The Testaments.' AFP

She late told reporters she was "happy to share it, I'm a sharing person," adding the prize money would go on paying off her mortgage.

Polyphony

Nominated for the 1986 prize, "The Handmaid's Tale" became an award-winning TV series in 2017, and sales of the English-language edition have topped eight million copies worldwide.

Evaristo's "Girl, Woman, Other" tells the tale of 12 women from black British families with roots across the country, Africa and the Caribbean in what judge Florence called a "polyphony".

He praised it for "giving voice to people who are not always articulated, of making the invisible visible."

Margaret Atwood and Bernardine Evaristo jointly win the Booker Prize. Reuters

Explaining the decision to break the rules, Florence said the "situation demanded" it.

"We explained this to the Booker people, they said they wouldn't allow it," he added.

"We spent an hour discussing it... we then took another half an hour, and still came to the conclusion that... this was our decision.

"They agreed to respect our decision."

Nigerian author Chigozie Obioma made the shortlist for "An Orchestra of Minorities" — his second novel after "The Fishermen", which was shortlisted in 2015.

Lucy Ellmann challenged judges with "Ducks, Newburyport" — a story made up almost entirely of one sentence that absorbs readers and is occasionally funny.

Agence France-Presse