The move is the latest tortuous step towards a potential end to western Europe's last major armed secessionist movement, once feared but now weakened by the arrests of many of its leaders.
In the declaration sent to Basque newspaper Gara, ETA said it had completed "dismantlement of the logistical and operational structures linked to the waging of the armed struggle" and was preparing to seal up its stashes of weapons.
Gara reported online on Saturday, along with advance excerpts from the declaration, that "a technical logistical structure is being created that aims to carry through to completion the sealing of the arms depots" held by ETA.
ETA, which is branded a terrorist group by the United States and European Union, said the move aimed to bring about "a transition from armed confrontation to democratic confrontation".
ETA is blamed for the killing of 829 people in a four-decade campaign of bombings and shootings for an independent homeland in parts of northern Spain and southern France.
In October 2011 it declared a "definitive end" to armed activity and in February this year international monitors released a video of weapons which it said ETA was putting out of use.
But the group has not formally disbanded as the Spanish and French governments demand.
There was no immediate formal response from Spain's government, which refuses to negotiate with ETA.
After its announcement in February, Spanish Prime Minister Mariano Rajoy said: "We do not have to give them anything in return for disbanding."
Madrid has notably refused to budge on one of ETA's most sensitive demands: the return of imprisoned ETA members to the Basque country.
ETA members have tried to gain concessions from the Spanish government over prison conditions, outraging victims' families.
In Gara's words, ETA said in its latest declaration that a settlement had to be reached "by overcoming all the consequences of the conflict, including its disarmament and the return home of prisoners and fugitives".
ETA has been weakened over recent years by the arrests of its senior leaders.
Only about 30 of its active members are thought to be still at large while hundreds are in prisons scattered around Spain and France.
Meanwhile non-violent leftist Basque nationalist parties have gained political influence and increasing power through regional elections in the region.
Such parties must now "take the leadership of the liberation process", ETA said in its new declaration.
It said it wanted to strengthen its political structures and dialogue "to move the peace process forward", according to Gara.
A group of five self-appointed monitors called the International Verification Commission, not recognised by Madrid, on released a video of black-masked members of the group presenting to monitors revolvers, a rifle, bullets and explosives.
Spain's conservative government was unmoved by the gesture while Spanish media derided the video as a "farce" and said the cache surrendered was ludicrously small.
Three days later ETA issued a statement via Gara saying it would put its whole arsenal "out of operational use".
ETA was formed in 1959 during the dictatorship of General Francisco Franco by a group of Basque nationalist students.
It carried out its last known deadly attack in 2009, when it killed two police officers by placing a bomb under their car on the Spanish holiday island of Majorca.