The European Court of Human Rights demanded Tuesday the speedy release of an ETA activist jailed for terrorist attacks and sanctioned Spain for extending her jail term by 10 years.
However, Spain said it would appeal the order by the Strasbourg-based court and vowed to keep Ines Del Rio Prada, who is serving several jail sentences for roles in the attacks, behind bars pending the appeal.
"The government disagrees with this decision and has asked state lawyers to lodge an appeal as soon as possible before the grand chamber" of the court, said Spain's Justice Minister Alberto Ruiz Gallardon.
Interior Minister Jorge Fernandez Diaz said that, pending the appeal, "her release is not going to be ordered because she would no doubt flee".
ETA is blamed for more than 800 deaths during a four-decade campaign of bombings and shootings for an independent Basque homeland. Last year it declared an end to its armed activity but has not formally disbanded.
The European court said Madrid had erred in Del Rio's case by retroactively applying a law on the accumulation of prison terms, saying the practice "is not allowed except when the legislative change favours the accused."
After eight sets of criminal proceedings for attacks, Del Rio was sentenced to various prison terms and began serving her sentence in 1989.
In November 2000, the various terms were fixed at 30 years, the maximum limit applicable at the time of the offences.
While she was serving her time, the Spanish high court in 2006 changed how sentences were commuted, saying that each individual sentence was henceforth to be commutted instead of the maximum one.
In April 2008, applying time off for work she did while in prison, the Spanish prison authorities then proposed that Del Rio be released in July 2008.
But before her release the high court asked that the release date be recalculated using the new procedures for commuting sentences, and a new release date was set for June 2017.
She appealed the decision, lost and then brought the case to the ECHR.
In its ruling on Tuesday, the rights court said it "emphasised that the domestic courts could not apply retroactively and to the detriment of the persons concerned the spirit of legislative changes that occurred after offences had been committed."
"The Court further held that Spain was to ensure the applicant's release at the earliest possible date," it said.
The two parties have three months to appeal Tuesday's ruling by asking the ECHR's Grand Chamber to examine the case.