Spain's central bank says Catalonia would be automatically ejected from the European Union and the eurozone if it were to declare independence.
Bank governor Luis Maria Linde said secession for economically powerful Catalonia would also close the door to European Central Bank financing.
Voters in Catalonia are to elect regional representatives on Sunday with pro-secession parties saying they will push for independence within 18 months if they win a majority in the 165-seat parliament, as most opinion polls predict they will.
Secession leaders insist a way may be found for an independent Catalonia to continue using the euro.
Spain has ruled out any possibility of the region becoming independent.
The region, whose capital is Barcelona, represents some 18% Spain's economic output.