But leaders of main parties on both right and left condemned the vote, unexpectedly supported by the political wing of the armed radical separatist group ETA, which is held responsible for 800 deaths in a long campaign of violence for Basque independence.
Basque leader Ibarretxe is a moderate who rejects the violence of ETA.
But Jordi Sevilla, Public Administration Minister in the national government in Madrid, called the plan "an absurdity, illegal and unconstitutional."
There would be dialogue, but no "government-to government" negotiations on the proposals, he said.
The mainstream parties fear the plan would mean the dismemberment of a country whose constitution, though recognising regional autonomy, stresses the indivisibility of the state.
Mariano Rajoy, lead of Spain's opposition right-wing Popular Party (PP), phoned Socialist Prime Minister Jose Luis Rodriguez Zapatero to offer bipartisan support against the plan.
"ETA has now got what it has been killing for during all these years," said PP Party Secretary-General Angel Acebes: "A radical nationalist project of rupture and fracture in Spain and the Basque country."
Ibarretxe's scheme was now tainted by association with ETA, the PP claimed.
"The Ibarretxe plan is henceforth the ETA plan too," said Acebes.
The plan must now go to a Basque referendum, and then be approved by the national parliament in Madrid.
But Jose Blanco, Secretary-General of the ruling Socialist Party (PSOE), predicted it would not get through the Madrid parliament.
Blanco said Socialists would vote unanimously against this "plan for discord and rupture that ignores the constitution," and which had succeeded in the Basque assembly due only to the votes of Batasuna, ETA's political wing.
But other regional nationalist movements welcomed it, with the Republican Catalans (ERC) of Catalonia in the north-east describing it as a democratic vote and the more moderate Catalan CiU party calling for respect for what it termed a legitimate vote.
The proposal was carried in the Basque assembly after a handful of pro-independence radicals shelved plans to reject it.
Three radicals helped to swing the vote for Ibarretxe's ruling coalition, led by his moderate Nationalist Basque Party (PNV) but which is two seats short of a majority in the 75-seat chamber.
Arnaldo Otegi, leader of the banned pro-independence party Batasuna (recently renamed Sozialista Abertzaleak), said during the debate his party's change of tack was not to help Ibarretxe but to kick-start a dialogue.