A German data protection body has ordered Facebook to end its policy of making members use their real names.
The policy violates German laws that give people the right to use pseudonyms online, said the data protection agency in Schleswig-Holstein.
The agency has issued a decree demanding that Facebook let people use fake names immediately.
Facebook said it would fight the decree "vigorously" and that its naming policy met European data protection rules.
"It is unacceptable that a US portal like Facebook violates German data protection law unopposed and with no prospect of an end," said Thilo Weichert, head of the regional data protection office in Schleswig Holstein, in a statement.
The ability to use a pseudonym on Facebook was "reasonable" said Mr Weichert and would allow people to use the service "without fear of unpleasant consequences".
Facebook's long-standing policy of making people use their real names did nothing to prevent people using the social network to throw insults or to hamper identity theft, he added.
Schleswig Holstein is piloting the action against Facebook and if it is successful the decision is likely to be adopted by the nation's other data protection agencies, it said, adding that the social network now has two weeks to oppose the decree in a German court.
The decree issued by the Schleswig Holstein office was "without merit" a Facebook spokeswoman told tech news site IT World adding that it planned to fight the order.