"Bernhard, a secret history" has revealed that the prince was a member of the German Nazi party until 1934, three years before he married Princess Juliana, the future queen of the Netherlands.
Annejet van der Zijl, a Dutch historian, has found membership documents in Berlin's Humboldt University that prove Prince Bernhard, who studied there, had joined Deutsche Studentenschaft, a National Socialist student fraternity, as well as the Nazi NSDAP and its paramilitary wing, the Sturmabteilung.
He left all the groups on leaving university in December 1934, when he went to work for the German chemical giant, IG Farben.
The prince always denied having been a member of the Nazi party, although he admitted that he briefly had sympathised with Adolf Hitler's regime. In one of the last interviews he gave before his death in 2004, he said: "I can swear this with my hand on the Bible: I was never a Nazi."