Nigerian author Chimamanda Ngozi Adichie says she was surprised to have won the Orange Broadband Prize for Fiction, despite being the favourite.
Adichie started writing at the age of six
"I immediately thought, that is the kiss of death," she told the BBC.
The 29-year-old becomes the youngest winner, and the first from Africa, to win for her novel Half of a Yellow Sun, set in the 1960s Biafran civil war.
Both her grandfathers died in that conflict and she said: "This book is my refusal to forget."
She beat five other contenders for the £30,000 women-only award, including Kiran Desai, shortlisted for her Booker Prize winner The Inheritance of Loss.
Chair of the judges Muriel Gray said Half A Yellow Sun was "a moving and exciting book by an incredibly exciting author."
"It's astonishing, not just in the skilful subject matter but in the brilliance of its accessibility."
At the ceremony, Adichie said: "I can only tell you how profoundly happy I am - Now I have to go and make a phone call to Nigeria."
Her book focuses on three characters - a houseboy, a rich Nigerian woman and an English would-be writer living in Nigeria and how their lives and relationships are affected by the war.
Some 1m people died in south-eastern Nigeria, when the region's Igbo-speaking people tried to secede.
CHIMAMANDA NGOZI ADICHIE FACTS
Born 1977 in Aba, south-east Nigeria
Educated in the university town of Nsukka
Masters degree in creative writing from Johns Hopkins University in the US
Hodder Fellow at Princeton University, teaching introductory fiction
Divides her time between Nigeria and the US
Half of a Yellow Sun takes its name from the flag of the short-lived Biafran Republic.
Adichie said she was proud to take on the mantle of Nigerian authors Chinua Achebe and Wole Soyinka.
She said there was a lot of writing talent in Africa, especially Nigeria and Kenya, but it was hard for new writers to flourish because of the poor infrastructure.
"We lack a system that encourages writing - when you think of an education system that devalues literature," she told the BBC.
She grew up on a university campus in the south-eastern Nigerian town of Nsukka and said she had been writing from the age of just six years old.
"I suppose I was born with the story-telling gene."
Adichie's first novel, Purple Hibiscus, was also shortlisted for the award - formerly known as the Orange Prize for Fiction - in 2004.
In 2002, she won the BBC Short Story competition for That Harmattan Morning.