The militant group Eta has resolved to disarm fully by 8 April, sources close to talks between the Basque separatists and Spanish government have said.
A pro-Basque independence and environmental group called Bizi has been given responsibility for the disarmament, according to one activist speaking to France's Le Monde.
The interior minister said Eta "won't get anything in exchange".
Eta declared a ceasefire in 2011 but is yet to relinquish its weapons.
The group killed more than 800 people over four decades of violence.
It was set up more than 50 years ago in the era of Spanish dictator General Franco to fight for an independent Basque homeland.
However, in recent years it has been sapped by the arrest of hundreds of members, including leadership figures, and the seizure of weapons caches.
"Eta has handed us responsibility for the disarmament of its arsenal and, as of the evening of 8 April, Eta will have completely handed over its weapons," Txetx Etcheverry, an activist with Bizi, was quoted as saying in Le Monde.
The 8 April date was also given by an unnamed source close to talks quoted by the AFP news agency.
No formal announcement has yet been made and the Spanish government has so far declined to make any detailed comment.
But in a tweet, Interior Minister Juan Ignacio Zoido said: "What Eta must do is dissolve itself and disappear. It has had time to disarm and it must know that it won't get anything in exchange."
In 2014 the International Verification Commission of international inspectors said Eta had taken some of its weapons out of action, but the Spanish government dismissed the move as "theatrical".
Spanish media say recent seizures of arms have been quite small, which they say indicate Eta does not have many weapons left - giving its disarmament a predominantly symbolic character.