Jean-Marie Loret, who died in 1985 aged 67, never met his father, but went on to fight Nazi forces during the Second World War.
His extraordinary story has now been backed up by a range of compelling evidence, both in France and in Germany, which is published in the latest edition of Paris's Le Point magazine.
Hitler is said to have had an affair with Mr Loret's mother, Charlotte Lobjoie, 16, as he took a break from the trenches in June 1917.
Although he was fighting the French near Seboncourt, in the northern Picardy region, Hitler made his way to Fournes-in-Weppe, a small town west of Lille, for regular leave.
There he met Miss Lobjoie, who later told their son: "One day I was cutting hay with other women, when we saw a German soldier on the other side of the street.
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