The ruling overturns a conviction and eight-year prison term handed down in 2017 when Ternera, a historic leader of the ETA group seeking independence for the Basque Country, was tried in absentia while he was on the run from the authorities.
Ternera, whose real name is Jose Antonio Urrutikoetxea Bengoetxea, headed ETA from 1977 to 1992 and is thought to have been behind a strategy of combining car bomb and shooting attacks in Spain in the 1980s.
The group’s attacks left at least 850 people dead in northern Spain and southwest France, an area considered the Basque homeland.
The group was formally dissolved in 2018.
In hiding since 2002, Ternera was convicted in absentia in France on charges he clandestinely took part in ETA’s operations from 2011 to 2013.
He was arrested in France in 2019 at a hospital in the French Alps where he was to undergo an operation.
Ternera, who appeared in court, will again face judges later this month on similar charges, of participating in ETA from 2002 to 2005, for which he had already been convicted on appeal in absentia to seven years in prison in 2010.
As both prison sentences were handed down in his absence, he was able to request a retrial for both.