Eneko Arronategui -- believed to be a member of the Basque separatist group ETA -- was arrested in July while living in Cambridge, England.
He faces trial in Spain for alleged terror-related offenses, including an attempt to assassinate Spain's King Juan Carlos in 1997, the Cambridge News reported.
A judge in Westminster Magistrates' Court in London had ruled in January Arronategui, 44, could be extradited on terror charges but not on the attempted assassination charge. The judge accepted the defense argument that the charge was unsustainable under Spanish law.
But two High Court judges overturned the ruling and said Arronategui could be extradited on all charges.
"The district judge should not have concerned herself with the issue of whether the appellant could be convicted in Spain of the offense of attempted assassination and so her decision must be quashed," said Griffith Williams, one of the judges.
Police have said Arronategui was part of a cell plotting to kill Carlos when he visited the Guggenheim Museum in Bilbao in October 1997.
The News said Arronategui is accused of shooting a policeman after the alleged attempt on the Spanish king's life.
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