The flyer, whose existence was first reported by the Sueddeutsche Zeitung on Saturday, parodies a national history competition and makes mocking references to Nazi concentration camps and the Holocaust.

Aiwanger, 52, denied authorship of the typewritten document, written in 1987 when he was 17,

The newspaper said multiple witnesses had said he was summoned before a school disciplinary committee over the flyer, which was found in a school lavatory.

"These are serious allegations. This flyer is hateful and revolting," said Bavaria's conservative premier, Markus Soeder. "The charges have to be cleared up, and fully."

Aiwanger heads the populist Free Voters party which governs Bavaria in coalition with Soeder's Christian Social Union (CSU). Soeder has previously said he wants to renew his alliance with the Free Voters after October's election.

Late on Saturday, Aiwanger issued a statement saying he had not written the flyer, a copy or copies of which were merely found in his schoolbag, and that the person who had done so, whose name he knew, would come forward of his own accord.

"Neither then nor now has it been my style to snitch on people," he wrote.

The leaders of the Social Democrats and Greens in Bavaria, said Aiwanger must stand down if he was the author of the flyer.

The election campaign in Bavaria, Germany's most prosperous large state, has been ill-tempered, with Aiwanger competing to sound tougher notes on immigration than Soeder, in one recent debate criticising the government for "throwing passports at those Syrians".

Opinion polls show that Soeder's CSU currently has 39% support. If it lost the Free Voters, which polls show currently commands 12% support, the party could be forced to turn to the much more left-wing Social Democrats or Greens, both of whom sit in the federal coalition in Berlin, to form a government.

Charges of antisemitism are particularly sensitive in Germany because of its role in the atrocities of the Holocaust.