Margallo said the last time such a situation happened was when Algeria declared independence from France in 1962 and its citizens lost French nationality and the right to European citizenship.
Catalan nationalists argue that the Spanish Constitution entitles those with Spanish nationality to keep it and that Catalans would be able to keep both their Spanish nationality and European passports.
Margallo`s warning was the latest against independence from Spanish officials as voters in the economically powerful northeastern region prepare to elect regional lawmakers Sunday.
Pro-secession parties say they will establish independence within 18 months if they win a majority of seats in the 165-seat regional parliament.
Most opinion polls predict they will win a seat majority but may fall short of a majority of votes, throwing the legitimacy of any eventual secession push into question.
On Monday, Spain`s Central Bank said Catalonia would be automatically ejected from the European Union and the eurozone if it were to declare independence. Spanish bank associations have also warned that banking groups might have to reconsider their positions in Catalonia in the case of independence.
Secession leaders insist that ways would be found for an independent Catalonia to continue using the euro.
The Spanish government has ruled out any possibility of the region becoming independent, saying secession would be unconstitutional.
Catalonia, whose capital is Barcelona, represents about 18 percent of Spain`s economic output.
Polls show Catalans overwhelmingly support the right for a secession referendum but are evenly divided over independence. Surveys show they oppose it if it means leaving the 28-nation EU.
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