MADRID — Catalonia, Spain’s economic powerhouse, headed toward a standoff with Madrid on Thursday after separatist parties in the northeastern region announced a date next year for an independence referendum, which the national government swiftly promised to block.

Artur Mas, the head of Catalonia’s regional government, said he would put before voters two questions next Nov. 9: “Do you want Catalonia to be a state?” and “Do you want that state to be independent?”

The threat to hold a vote — and potentially break apart Spain — drew an immediate rebuke from Madrid and once again pushed to the political fore one of the thorniest issues that has confronted the conservative government of Prime Minister Mariano Rajoy.

Immediately after the announcement, Alberto Ruiz-Gallardón, Spain’s justice minister, warned Mr. Mas that such a vote would violate the Spanish Constitution and that the government in Madrid would not allow it to happen. “The poll will not be held,” he said.