The Irish writer, 28, said it was "an enormous privilege" to win both the top prize and fiction book of the year.

Head judge Alice O'Keeffe praised Obama's entry, but deemed Rooney's work "exceptional" and "profoundly moving".

The former first lady's memoir, Becoming, did however clinch the non-fiction narrative and audiobook awards.

The awards, dubbed the Nibbies, are organised by trade magazine The Bookseller.

The winners from each of the British Book Awards' eight categories go forward for the overall best book of the year.

Each category has its own judging panel.

Rooney's winning novel, which charts an on-off relationship between two college students, cements her meteoric rise.

It follows her critically-acclaimed debut, 2017's Conversations with Friends, that saw her named Dunlop young writer of the year.

Judges said Becoming - chronicling Obama's life from the south side of Chicago to the White House - stood "head and shoulders above the competition" in the narrative non-fiction category.

It beat Michael Wolff's White House expose Fire and Fury to take top spot.

The audiobook version, narrated by Obama herself, beat Ben Whishaw's telling of Stephen Hawking's final book, Brief Answers to the Big Questions, and Robert Galbraith's Lethal White, narrated by Robert Glenister.

Becoming is already the best-selling audiobook of all time.

Obama said it was "an incredible honour" to have received her award.

"It's been such an uplifting an powerful experience to share my story with everyone across the United Kingdom these past few months," she added.

"Two years ago, when I started writing this memoir, I wasn't thinking about awards; my biggest hope was simply to create something meaningful for the people who read it, something they might be able to connect to their own lives. Because I know that my story isn't unique.

"It's the story of a working class black girl learning to make music on the piano with broken keys, of a high school student who wondered if she's good enough, of a mother trying to balance a career, two daughters and a husband with big goals, while carving out a better sense of herself."

Ice Monster success

French-Moroccan writer Leila Slimani won debut fiction book of the year for Lullaby - a dark tale of infanticide that lost none of its power in translation.

The London ceremony also saw thriller writer Lee Child named author of the year and veteran children's author Judith Kerr named illustrator of the year.

The awards also recognise stand-out books released for younger readers.

David Walliams' The Ice Monster took home children's fiction book of the year, beating entries including Jacqueline Wilson's My Mum Tracy Beaker and Nigerian-American writer Tomi Adeyemi's Children of Blood and Bone.

Judge Tina Gaisford-Waller, of Hunting Raven Books, described it as "gloriously, snortily funny, warm, wacky, wise, and written wholeheartedly for the reader".

Walliams is also a judge on Britain's Got Talent

The children's illustrated and non-fiction category - newly introduced this year to reflect the growing importance of illustration in children's books - went to You Are Awesome by Matthew Syed, illustrated by Toby Triumph.

Philip Jones, editor of The Bookseller, expressed delight at the winners for showing off the "breadth and dynamism" of British talent.