Monique Roffey has won the £30,000 Costa book of the year award for her sixth novel, The Mermaid of Black Conch, which opens as a fisherman on a Caribbean island sees a “barnacled, seaweed-clotted” mermaid raise her head from the sea.

Suzannah Lipscomb, the historian and broadcaster who chaired the judges, said the novel was “utterly original – unlike anything we’ve ever read – and feels like a classic in the making from a writer at the height of her powers”. Based on a legend from the Taino, an indigenous people of the Caribbean, the novel is a dark love story about fisherman David and Aycayia, a beautiful woman cursed by jealous wives to live as a mermaid, who has swum the Caribbean for centuries.