Madrid is seeking to stop the Catalan government spending public money on its planned independence referendum.
In a boost for the credibility of the referendum, the mayor of Barcelona said earlier on Thursday that the vote would go ahead in the city, having previously expressed concern that civil servants involved may lose their jobs.
In a letter dated September 13, Catalan Deputy Governor Oriol Junqueras told Spanish Budget Minister Cristobal Montoro the region would no longer comply with the obligation to submit its accounts every week.
"If anyone urges you to go to a polling station, don't go, because the referendum can't take place, it would be an absolutely illegal act", the prime minister said.
This new system will allow the Spanish state to "replace the region for the majority of essential spending", he added.
The region, with a population of 7.5 million, is regarded by Spain as an autonomous community and its people a nationality, in recognition of the area's language and cultural distinctions.
"British companies are wary of any political instability..." Once again this week, Rajoy has rejected sanctioning a self-determination vote, telling reporters "there cannot be a referendum and it would be an absolutely illegal act".
But the overture was roundly rebuffed by the spokesman for the Spanish government, Íñigo Méndez de Vigo, who said Madrid had not received the letter and only learned about it through the press.
"We are acting firmly to ensure the rights and liberties of everyone and will react to whatever the secessionists do", he said.
But a big question mark remains over Barcelona, Catalonia's biggest city run by Mayor Ada Colau, a left-wing former activist.
"They've lost the plot", said Albert Batet, mayor of the town of Valls and one of those summoned for questioning.
Spanish Prime Minister Mariano Rajoy was taking the unionist cause directly to Barcelona on Friday, addressing a meeting of his Popular Party in the Catalan capital. An open dialogue without conditions. A political dialogue, based on the legitimacy we all have, to make possible something that in a democracy is never a problem and even less a crime: "listening to the voice of the people", the officials wrote in the letter.
The letter accused the Spanish state of "an unprecedented repression offensive".
Madrid has the constitutional power to take over a regional government or send in the police to force Catalonia to drop the vote, but either step would rock Spain's decentralised model of government where power is devolved to 17 self-governed regions.
Jean-Claude Juncker, the European Commission President, indicated yesterday that the European Union would recognise the independent state of Catalonia if the referendum voted yes.