ETA, the Basque separatist group that killed around 850 people during its half-century campaign for independence from Spain, announced its full dissolution, Spanish media reported.
The declaration was made in a letter dated April 16 and sent to a number of political institutions, Spanish news agency EFE reported. The announcement had been expected, and came just over a year after the group handed over its weapons, following a 2011 ceasefire.
ETA, which stands for "Euskadi Ta Askatasuna" or "Basque homeland and freedom," was founded in 1959 in response to the frustration felt by Basques during the repressive regime of General Francisco Franco. Its aim was to gain independence from Spain and establish a state in the north of Spain and southwest France.
The group released a statement last month in which it apologized to the families of its victims, adding that it accepted "direct responsibility" for its crimes.
"We know that we caused a lot of pain during that long period of armed struggle, including damage that can never be put right," it said.
"We wish to show our respect for those who were killed or wounded by ETA and those who were affected by the conflict. We are truly sorry."
The Spanish government said it would continue to pursue members of ETA, despite its self-declared dissolution. "The Spanish government has always maintained its position: ETA members didn't achieve anything for stopping killing people and they won't achieve anything for a declaration that they call 'dissolution,'" Spanish Interior Minister Juan Ignacio Zoido told the Spanish news agency EFE. "Spanish security forces will continue chasing the terrorists wherever they are."
ETA carried out a number of political assassinations and bombings during Franco's regime, which it continued after the dictator's death.
The group's appeal has waned in part because of a rejection of its violent tactics.
The 1978 Spanish constitution also ensured the region was given more autonomy, including its own police force, language, education and fiscal rights.
In 1973 the group assassinated Spanish Prime Minister Luis Carrero Blanco in a car-bomb attack.
ETA also caused outrage in 1987 after the car bombing of a Barcelona supermarket killed 21 people, including a pregnant woman and two children, drawing condemnation from across the world.