Germany’s far-right populist Alternative für Deutschland (AfD) faces a serious split after a row over anti-Semitism prompted 13 MPs – including one of the AfD’s two co-leaders – to walk out of the party.

Tuesday’s split, the second in the party’s three-year history, was triggered by the writings of Wolfgang Gedeon, an AfD parliamentarian in the state parliament of Baden-Württemberg in Stuttgart.

He faced accusations of relativising the Holocaust for belittling the Nazi-era murder of six million Jews as “some heinous acts” and dubbing Holocaust deniers “dissidents”.

The AfD has been at war since the passages emerged last month. Prof Jörg Meuthen, the conservative co-leader of the AfD and head of the Stuttgart parliamentary party, wanted to eject Mr Gedeon immediately. But he was blocked by Frauke Petry, the party’s populist co-leader from the eastern state of Saxony.

Matters came to a head on Tuesday when two expert opinions commissioned on Mr Gedeon’s writings came to the same conclusion: his texts were irredeemably anti-Semitic. After Prof Meuthen failed to get the necessary two-thirds majority of MPs to expel Mr Gedeon from the 23-member parliamentary party, the AfD leader walked out – taking 12 other MPs with him.

“We will create a party that is anti-Semitism-free and I hope more will join us,” he said. “We are not sitting here as any revolutionaries who want to form a new party. We are the AfD, we stand for this party.”

The AfD was founded during the euro crisis as an anti-bailout movement, but last year ousted its founder, economics professor Bernd Lucke, in favour of Ms Petry and Prof Meuthen.

They took a hard line against mass migration that Prof Lucke and others have attacked as racist and xenophobic.

From a standing start, the AfD missed entering the Bundestag in 2013 but now sits in half of Germany’s 16 state parliaments. With double-digit support in national opinion polls, the AfD’s hopes of entering the Bundestag on the second go next year may yet stumble over the case of Wolfgang Gedeon, a 69-year-old doctor and part-time author.

Dresden political scientist Werner Patzelt, one of two academics commissioned to study Mr Gedeon’s writings, said his texts displayed “clear anti-Jewish tendencies which tip over into anti-Zionism and secondary anti-Semitism, along the lines of ‘the Jews have only themselves to blame’”.

In his texts, Mr Gedeon described the Holocaust as “the West’s civil religion” and cited among his sources the 1903 “Protocols of the Elders of Zion”, a plan for supposed Jewish world domination that was discredited as a fake almost a century ago.

“Gedeon started as a Maoist and communist then shifted to the hard right,” said Prof Patzelt. “He’s a not untypical case of someone who spends their lives on the political fringes, the kind of loose cannon that a protest party like the AfD always attracts.”

Prof Patzelt sees the Stuttgart row as being less about anti-Semitism than a proxy battle over who should lead the AfD in next year’s federal election: Ms Petry and her eastern German populist camp or the western German camp around the conservative economist Prof Meuthen.

On Tuesday afternoon, with Ms Petry reportedly hurrying to Stuttgart, the rest of the AfD’s board said they would back Prof Meuthen and his breakaway group as the official parliamentary party in Stuttgart.

Germany’s political establishment could barely contain their glee on Tuesday. Hans-Peter Friedrich, an MP with Bavaria’s Christian Social Union (CSU), attacked the AfD as “attention-seekers and egomaniacs who are no alternative for anything or anyone”.