Germany's far-right Alternative for Germany (AfD) party won a legal victory against the country's domestic spy agency on Tuesday.

The party complained that the Office for the Protection of the Constitution (BfV) had defamed the party by publicly treating it as a "test case."

A test case classification means the BfV can use open-source material to investigate whether the party poses a threat to the constitutional order. It does not allow the BfV to use undercover agents or other intelligence methods to collect information. Authorities can declare a party a test case if they detect the first signs of extremism.

However, the Administrative Court of Cologne ruled that the BfV had failed to justify the designation, relying only on "fragments of suspicion."

Read more: Germany's domestic spy agency and its history of scandals

The BfV announced its classification in a public press conference in January. The court found that the term "test case" had a "negative effect" on the public.

'Politically motivated instrumentalization'

AfD party leader Jörg Meuthen said: "The decision clearly demonstrates that the actions of the (BfV) and in particular of its President (Thomas Haldenwang) are not in accordance with the principles of the rule of law". Thus, he continued, its "politically motivated instrumentalization" has failed for the time being.

Controversial spokesman Björn Höcke called on the head of the BfV to resign, saying he had "ruined the reputation of the entire Office for the Protection of the Constitution."

The AfD's Roland Hartwig called it "extremely alarming that the Federal Office, as the guardian of the constitution, is itself breaking the law."

Read more: Germany's far-right AfD files lawsuit against spy agency

Benjamin Strasser, from the business-friendly FDP party said: "Even if the (BfV) is no longer allowed to call it that, the AfD remains a test case for democracy."

The decision can still be appealed.

aw/amp (AFP, dpa, Reuters)