"'Disarmament day' is tomorrow and we want to warn that still the process can be attacked by the enemies of peace," the group said in a letter published in English and Spanish and dated April 7.
In the note, addressed to the international community and published by the BBC yesterday, ETA said it had given up "all its weaponry (arms and explosives) to Basque civil society representatives" and described itself as a "disarmed organization".
The group said Bayonne, a city in the French Basque region, will be the focal point of the disarmament process where it expected thousands of people to gather on Saturday.
Urkullu at the time called on the Spanish and French governments to "show ambitious vision and open direct lines of communication" with ETA.
More From This Section
But Madrid, a strong opponent of ETA, rebuffed the plea and instead demanded the group "dissolve" and never reappear.
In its newly-published letter, ETA said the process of disarming has been "a hard and difficult task", praising the Basque authorities while accusing Spain and France of being "stubborn".
It gave up its armed struggle in 2011 but has been seeking to negotiate its dissolution in exchange for amnesties or improved prison conditions for roughly 350 members of the group being held in Spain and France.
Around 100 are serving sentences of more than 10 years.