Previous secessionist challenges in Catalonia - a populous wealthy region whose capital is Barcelona and which has its own language - were blocked by Spain's conservative government and the Constitutional Court.
"The question will be: 'Do you want Catalonia to become an independent state in the form of a republic'," Carles Puigdemont, president of the government of Catalonia, said.
He said attempts to agree a date and the wording of the question with the Madrid government - which is vehemently opposed to allowing Catalonia to split from Spain - failed and left him with no other choice than moving unilaterally.
"We have always made very diverse offers and all of them have been rejected without any exception," Puigdemont said.
Pro-independence campaigners staged a symbolic ballot, organised by volunteers rather than government officials to get around court restrictions, in 2014, months after Scots voted to stay in the United Kingdom.
Some 2 million people voted in favour of secession in that non-binding ballot, though turnaround was relatively low.
Under Article 155 of Spain's constitution, Madrid has the power to intervene directly in the running of Catalonia's regional government, forcing it to drop the vote.
This could involve sending in the police or suspending the regional government's authority to rule.
This is widely seen as a last resort move, however, and many analysts believe the clash will instead culminate in regional elections in Catalonia.
(Additional reporting by Raquel Castillo in Madrid; Writing by Julien Toyer; Editing by Robin Pomeroy)
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