In the memo, published Sunday by the Gara newspaper in San Sebastian, Spain, the terrorist group's executive committee remained committed to independence for Spain's Basque country and announced no moves to dissolve itself or to establish a timetable for disarmament, as sought by Madrid.
But it also confirmed the abandonment of armed struggle as a strategic move "to take practical steps" toward achieving its goals.
The committee, the memo says, remains overwhelmingly in support of ETA's 2011 announcement of the "definitive cessation" of its armed activity.
Gara reported the leaked document had been seized by the Spanish Civil Guard this month during the arrests of eight members of the EPPK, a collective set up to help Basque prisoners.
The San Sebastian newspaper said the seized document indicates that more than 80 percent of ETA members had voted to support the cease-fire, with only 4.2 percent against, and that it suggests the decision goes beyond being a mere negotiating ploy.
The conservative government of Prime Minister Mariano Rajoy and ETA leaders have never entered into official talks and some 40 alleged Basque terrorists have been arrested since December 2011, dealing its leadership a potentially crippling blow.
The group has been blamed for the deaths of 829 people in a campaign of violence, kidnappings and bombings over a 40-year period in northern Spain and southern France. Some 700 Basque militants are imprisoned.
ETA leaders have long sought a general amnesty for them, but in December indicated they could instead accept "legal channels" for their release.
The European Court of Human Rights ruled in October that Spain couldn't apply a law exempting ETA members from early release retroactively before 2006, meaning at least 56 of the prisoners had to be freed.
Spain had used the law to justify the continued imprisonment of ETA figures despite earning time off for good behavior.
The release of several ETA prisoners last year triggered angry scenes and protests in Madrid and elsewhere in the country, putting pressure on the government to remain firm in dealing with Basque prisoners.
Spanish Interior Minister Jorge Fernandez Diaz, in an interview published Sunday by the daily newspaper ABC, said the memo indicated ETA's strategy is to transform itself from a terrorist group into a "political agent."
"ETA has not raised the possibility of its dissolution, but wants to perpetuate itself as a political agent more involved in what they call the 'release process' to achieve their goals and maintain their strategic objectives: independence and socialism," he said. "It is clear that we will not allow that."
Spain's prison policy, Diaz said, "will not change. The government's anti-terrorism policy will remain firm until the dissolution of ETA. And the prison policy is a cornerstone of our anti-terrorism policy."
Spain's opposition Socialist Party, meanwhile, urged ETA to issue "the only statement that would be of any interest: Its dissolution and the final delivery of its weapons."
Inigo Urkullu, the president of the Basque government, said Sunday the memo did not represent a "step forward" because the decision to abandon violence by ETA in 2011 had already been declared "final and irreversible," Europa Press reported.
Instead, he also reiterated the need for disarmament and dissolution of ETA.
"Disarmament and demilitarization as an organization, and recognition of the damage and suffering caused by the conflict, are the steps you need to see from the Basque ETA," Urkullu said.