The designation is certain to inflame debates over whether the party should be banned, though some polls show it to be the most popular in the country.

Germany's spy agency on Friday classified the far-right Alternative for Germany (AfD) as "extremist", enabling it to step up monitoring of the country's biggest opposition party, which decried the move as a "blow against democracy".

The AfD came second in February elections and has a record number of seats in the new parliament.