The Audiencia Nacional’s decision follows a European court ruling which overturned Spain’s extension of Eta prisoners’ sentences.
Last month there was bitterness among relatives of ETA victims when Eta member Ines del Rio was released.
A violent 40-year campaign by ETA killed more than 800 people. The group declared a ceasefire in 2011.
Spain and some of its Western allies, including France and the UK, consider Eta a “terrorist” organization.
On October 21, the European Court of Human Rights upheld its 2012 ruling against Spain which opposed a legal practice known as the “Parot doctrine” – that allowed prison terms to be extended for people convicted of terrorist offences.
The next day, Spain released Ines del Rio, who was serving a 30-year prison sentence for her role in bomb attacks in the 1980s.
The court decision on Friday was narrowly in favor of releasing nine prisoners and keeping two others behind bars. Nine judges were in favor, and eight were against.
The nine militants to be freed include Domingo Troitino, jailed for a 1987 attack on a supermarket in Barcelona which killed 21 people.
Domingo Troitino is the brother of Antonio Troitino, who was released from custody in London last month in line with the European court ruling.
The other eight are: Jokin Mirena Sancho, Isidro Maria Garalde, Jose Ignacio Urdiain, Jon Koldo Aginagalde, Joseba Koldobika, Elias Fernandez Castanares, Juan Francisco Gomez Lopez and Luis Maria Azkargorta.
A Spanish ruling in March 2006 meant that the most serious criminals in Spain could not qualify for early release.
But the European Court in Strasbourg said the ruling must not apply retroactively and so, it argued, dozens of Eta prisoners and other criminals convicted before that date should be released.
The Strasbourg ruling has been condemned by organizations in Spain representing the victims of Eta bombings and murders.
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