Remains of Captain Fergus Bowes-Lyon, her mother's brother, have never been recovered since he died at the Battle of Loos in northern France in 1915.
The Queen spoke of detective work to track down his suspected mass grave in 2011 as she met UK Ministry of Defence officials who help families find the bodies of their loved ones, The Daily Telegraph reported.
They told her that their workload has kept increasing amid growing interest in family history and the increasing ability of people to research the lives of their ancestors online.
The Queen responded, "Well yes, I think some of their work led to the discovery of where my mother's brother fell."
Fergus, 26, a Captain in the Black Watch regiment, died leading an attack on the German lines at the heavily fortified Hohenzollern Redoubt.
His leg was blown off by a barrage of German artillery and bullets then struck him in the chest and shoulder.
His death, followed by their inability to find out where his remains lay, hit the Queen Mother and her family hard, the daily said.
In 2011 Captain Bowes-Lyon's grandson, James Voicey-Cecil with the help of his second cousin, Prince Charles, and historian Christopher Bailey, helped trace where he was buried in a mass grave in a quarry.
In the past year a small team of two has become a commemorations unit of seven people tracing bodies going back to the First World War and currently dealing with a backlog of 60 cases.
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