Germany's far-right Alternative for Germany (AfD) has expelled Doris von Sayn-Wittgenstein, the party leader in the northern state of Schleswig-Holstein.

A party tribunal found that the 64-year-old had damaged the party's reputation by supporting Gedächtnisstätte ("Memorial Site"), an organization accused of Holocaust denial.

Gedächtnisstätte is on a list of organizations the AfD says are incompatible with its own principles, and is also on the watch list of several domestic intelligence agencies in Germany. The organization was founded in 1992 by Ursula Haverbeck, one of Germany's most notorious Holocaust deniers, who is currently in prison following repeated offenses.

Sayn-Wittgenstein still enjoyed the support of Schleswig-Holstein's regional party only two months ago, when she was re-elected party leader, but prominent AfD politicians on Thursday claimed the expulsion as a sign of their stand against extremism.

"The decision of the federal party tribunal sends an important signal both to the AfD and to the public," said Jörg Nobis, the party parliamentary leader in Schleswig-Holstein. "For members of the AfD there is a red line, crossing which always leads to ending the membership."

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Kay Gottschalk and Georg Pazderski, both members of the AfD's national party council, said this was "a good day for the AfD."

Sayn-Wittgenstein has said she will appeal the decision and that she still considers herself the state's party leader, a claim the AfD has rejected. "Without the rights of a party member she is not allowed to carry out her duties," party spokesman Bastian Bafd ehrens said.

The state party leader has long been considered one of the AfD's most controversial figures. Though she previously admitted to actively supporting Gedächtnisstätte in 2014, she subsequently distanced herself from the group.

She also denied ever having been a member of the group, though the AfD has said she once claimed she was at a party meeting. Speaking to the DPA news agency on Wednesday, she indirectly accused the national party of hypocrisy. "I'd like to know how the national leadership positions itself towards far-right extremism in general," she said.

Several other prominent AfD leaders, most notably Björn Höcke, head of the parliamentary party in Thuringia, have been accused of maintaining close ties with the far-right.