The Basque separatist group Eta says its ceasefire with the Spanish government will end at midnight.
Prime Minister Jose Luis Rodriguez Zapatero condemned Eta's move. "Eta's decision goes totally in the opposite direction of the path that Basque and Spanish society want, the path of peace," he said.
Eta declared a "permanent" ceasefire in March 2006, and had insisted it still held despite a bomb that killed two people at Madrid airport in December. But in a message printed by the Basque newspaper Berria on Tuesday, the banned group said "minimum conditions for continuing a process of negotiations do not exist".
After the Madrid airport attack Spain's Socialist government broke off peace talks. Eta said that from Wednesday it would defend the Basque country "with weapons and on all fronts". The announcement suggests that another big attack could be imminent, observers say.
The group has killed more than 800 people in its four-decade campaign to set up an independent state in northern Spain and south-western France. Despite last December's attack, Eta's activities have been waning, with the number of bombings falling in recent years.
In the past few days, Eta has sent letters to Basque businesses urging them to help finance "the liberation and construction of Euskal Herria (the Greater Basque Country)", Spanish media report.