A decades-old theory that Adolf Hitler had a Jewish grandparent has been reanimated by a new study, although mainstream historians cast doubt on the claims.

In an article for the Journal of European Studies, Dr Leonard Sax, a psychologist, claimed to have evidence to challenge a key argument against a post-war claim by a high ranking Nazi that Hitler’s paternal grandfather was Jewish.

Hans Frank, who held the position of Governor-General of Poland during the Holocaust, wrote his memoirs shortly before his execution in 1946. Frank, a former lawyer, claimed that in 1930 Hitler had asked him to look into claims that his grandfather was Jewish, after alleged blackmail claims from a distant relative. Frank said his research had found that Hitler’s grandmother, Maria Schicklgruber, had given birth to his father Alois out of wedlock while working as a cook for a Jewish family, the Frankfurters,  in the Austrian city of Graz. He also claimed to have found evidence of payments from Leopold Frankfurter, the head of the household, to Maria, after she had left his employ.

Holes in Frank’s account include the fact that Jews were expelled from Graz in the fifteenth century and not permitted to return until the 1860s, more than two decades after Alois’s birth, the lack of any record of a “Leopold Frankfurter” living in Graz in the 1830s and inaccuracies concerning where Hitler’s grandmother came from.

However, Dr Sax claims to have evidence supporting the existence of a small Jewish community in Graz by at least 1850, going as far as to suggest that his paper revealed “new evidence that Adolf Hitler’s grandfather was Jewish” and positing that a factor driving Hitler’s antisemitism “was his intense need to prove he was not Jewish.”

However, there is still no evidence of a Jewish community of any sort in Graz in the mid-1830s, when Alois Hitler was born – and serious historical scholars have long dismissed the Frank claim, including Ian Kershaw, one of the world’s leading experts on Nazism and Hitler.

Speaking to the Times of Israel, Sir Richard Evans, former Regius Professor of History at Cambridge and author of the Third Reich Trilogy, said that “even if there were Jews living in Graz in the 1830s, at the time when Adolf Hitler’s father Alois was born, this does not prove anything at all about the identity of Hitler’s paternal grandfather.”

He also told the paper that there was no contemporary evidence that there was a Jewish family called Frankenberger who lived there - "there was a family in Graz called Frankenreiter but it was not Jewish." Similarly, "no correspondence between Hitler’s father or paternal grandmother has ever been found. Nor is there any evidence for Frank’s claim that Hitler’s half-nephew knew about it and was blackmailing Hitler, as Frank claimed.”

One theory about the Frank claim was that the war criminal, who turned against Nazism after the fall of the regime in 1945 but who remained a committed antisemite, was seeking to present Hitler as having Jewish roots as a way to try and pin the disaster of Nazi Germany on the Jews themselves.