Andrea Levy previously won the Orange Prize

Small Island is set in post-war London and centres on a landlady and her lodgers, who include a Jamaican adapting to life outside the forces.

The book previously won the Orange Prize for fiction, and had been 5/4 favourite for the Whitbread prize.

The judges said Levy's work was a "clear winner".

Levy said that "in her wildest dreams" she had never expected to win the prize, and later added that the prize has given her novel the "stamp of approval".

It is a brilliantly observed novel of a period of English history that many people seem not to know very much about Sir Trevor McDonald

"I didn't think my luck could hold out for this long and I'm just so thrilled that people like the book."

She said her heart sank when bookmakers made her the favourite, because "favourites never win".

She used her acceptance speech to praise those who work for good race relations in Britain, before adding that she had no plans to go wild with her prize money.

"I'm just happy this money will enable me to write a new book," she said.

'Great story'

The judging panel was chaired by newsreader Sir Trevor McDonald and also included actor Hugh Grant and writer Joanne Harris.

Sir Trevor said the book, Levy's fourth novel, was a "great story" which she had "handled brilliantly".

Hugh Grant was this year's celebrity judge on the Whitbread panel

"It is a brilliantly observed novel of a period of English history that many people seem not to know very much about," added Sir Trevor.

The five books to go forward for the main prize all won their respective category - first novel, novel, biography, poetry and children's book - on 6 January.

Actor Hugh Grant was a member of the judging panel, and said the experience has rekindled his interest in literature.