Beating over 122,000 entries from 170 countries, the judges voted unanimously for Gjestvang’s impactful images which captured the young survivors with 'immense sensitivity and honesty'.
Killer Anders Behring Breivik carried out a mass shooting at a camp of the Workers' Youth League of the Labour Party on the island, where he killed 69 people, mostly teenagers.
Chair of the Honorary Jury, Catherine Chermayeff, Director of Special Projects at Magnum Photos, said: “As the Jury Chairman I am delighted that Andrea Gjestvang has won the L'Iris d'Or prize. The entire jury was unanimous in selecting this body of work.
"One Day in History" is a quiet, thoughtful and ultimately powerful voice for the children and survivors of the massacre in Norway. We were all moved by the dignity and beauty of these images."
In just a few years, Andrea Gjestvang has established herself as one of the world’s most exciting young photographers.
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Her work has been published in Newsweek, M Le Monde, Stern and D Republica, and exhibited worldwide.
Most recently she was awarded the Photo of the Year prize at the 2012 Norwegian Picture of the Year competition for her portrait of Utøya survivor Ylva Helen Schwenke.
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Last night she scooped $25,000 in prize money at a ceremony in London.
The L’Iris d’Or winning series of work, alongside those of the other winners announced at the awards, will be displayed as part of the 2013 Sony World Photography Awards Exhibition at Somerset House from April 26 – May 12.
The images will also be published in the 2013 edition of the Sony World Photography Awards book, available to buy here.