The Spanish supreme court has approved a government request to permanently ban Batasuna, seen as the political wing of Basque armed separatist group ETA.
A three-year suspension was imposed on the radical separatist party in August 2002 under a controversial law on political parties on the grounds that Batasuna was part of "ETA's terrorist network".
Today's decision to outlaw Batasuna permanently could prevent it putting up candidates in May for local elections in the Basque country and for regional and local elections in the neighbouring province of Navarra, which has a large Basque-speaking community.
Batasuna regularly obtains between 10 and 20 per cent of the vote in Basque regional elections. It has seven seats in the parliament of the autonomous region.
The right-wing government began seeking a permanent ban on Batasuna in September 2002 with the overwhelming support of the parliament in Madrid.
The supreme court said the 16 judges considering the case had voted unanimously to outlaw Batasuna after four days of deliberations. The permanent ban will enter into force as soon as the party is officially informed of the verdict.
The court also banned the two radical separatist Basque parties that had preceded Batasuna - Herri Batasuna and Euskal Herritarrok - under the same controversial law on political parties.
Batasuna, which submitted a 150-page document in its defence to the court, has the right to challenge the ban in the country's constitutional court. In October it lost an appeal to have the temporary ban on it overturned.
ETA has been fighting for over three decades for an independent Basque homeland comprising parts of northern Spain and southwestern France.