ETA, founded in 1959 and considered a terrorist group by the European Union, has been seeking to negotiate its dissolution in exchange for amnesties or improved prison conditions for its roughly 350 members held in Spain and France.
The group is blamed for the deaths of 829 people in a four-decade campaign of bombings and shootings for an independent Basque homeland, which straddles northern Spain and southwestern France.
The separatist group has been severely weakened in recent years after hundreds of its members were arrested and some of its weapons were seized.
On Friday, France's Le Monde reported that ETA were set to announce plans to disarm and have scheduled a full handover of weapons for April 8. Several Basque political leaders said this was credible.
Basque independence sympathizers have lobbied for years for imprisoned ETA operatives to be moved to prisons closer to the Basque Country.
"ETA has made the umpteenth announcement and says it will disarm ... it won't be in exchange for anything," Rajoy said during a meeting of his People's Party in the Basque Country.
The group has attempted to broker deals in exchange for disarmament on several occasions but the Spanish and French governments have refused to work with them, insisting only that they hand over their arsenal.
At the end of 2015, there were 403 people detained in 74 prisons throughout Spain, according to Etxerat, an association which represents families and associates of the group.