"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."

As the Nazis invaded Holland in 1940, the young prince consort, then aged 28, organised a group of palace guards that engaged in combat with German forces.

Denied the chance to stay and organise resistance by the then Queen Wilhelmina, he became head of the Dutch Royal Military Mission based in London.

As "Wing Commander Gibbs" – an honorary rank he held in the RAF – the prince later flew Allied bombing raids over occupied Europe before returning in 1944 as a Dutch war hero