A German historian claims to have discovered written proof verifying the myth that Adolf Hitler had only one testicle.

In a new book published on Friday, Prof. Peter Fleischmann of the University of Erlangen says that he found proof of the German dictator's undescended testicle in a file from Landsberg Prison, where Hitler was jailed after the abortive Beer Hall Putsch in November 1923.

The file contains the report of a medical examination conducted on Hitler after his arrest by Dr. Josef Brinsteiner, Landsberg Prison's medical officer. The report records: “Adolf Hitler, artist, recently writer,” as being “healthy and strong” but suffering from “right-side cryptorchidism.”

The latter is a condition that occurs when one or both testicles fail to descend during puberty. In the case of Hitler, it was the right testicle that failed to descend.

The book, ”Hitler as a prisoner in Landsberg am Lech 1923/24," was revealed to the German media on Friday for the first time.

The records of the medical examination were long thought lost, until they surfaced at an auction in 2010. They were promptly confiscated by the Bavarian government and have only now been properly studied.

Rumors about the dictator's missing testicle have circulated ever since his rise to power and British schoolchildren have for decades sung the line "Hitler has only got one ball" – a wartime song that mocked the German tyrant.

The morale-boosting ditty emerged at the beginning of World War II as part of an effort to ridicule the Fuhrer and his fellow leaders.