Germany is challenging one of Facebook's golden rules.
The BBC reported the data protection agency Unabhaengiges Landeszentrum fuer Datenschutz (ULD) in the northern German state of Schleswig-Holstein told Facebook it must end its long-standing policy which forces members to use their real names.
The German state of Schleswig-Holstein said the policy violates the country's law that gives "people the right to use pseudonyms online," according to the BBC.
CNET wrote the northern German agency said the law guarantees the "fundamental right to freedom of expression on the Internet."
Facebook responded it will "vigorously" fight the decree.
"It is unacceptable that a US portal like Facebook violates German data protection law unopposed and with no prospect of an end," said a statement from Schleswig Holstein's Thilo Weichert, who runs the company's regional data protection office.
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According to Schleswig Holstein, the decision if upheld in German court, could be adopted by the country's other data protection agencies throughout Germany, the BBC said.