On Friday, anti-terror judge Baltasar Garzon banned a weekend rally at which a new Basque separatist political party was to have been unveiled.
Garzon said the new party was too closely linked to Batasuna, a group already outlawed for its ties with the Eta.
The judge said documents presented by the proposed new party, Abertzale Sozialisten Batasuna, "contain several elements that indicate the political group is an apparent continuation of Batasuna", which was made illegal in 2003.
The new party, whose name in Basque means Patriotic Socialist Union, was registered by members of Batasuna on Tuesday in the hope that it could field candidates for municipal elections due to take place in May.
On Wednesday, Spain's interior ministry began moves to ban the new party.