Spanish prime minister Mariano Rajoy has accused Catalonia's president Carles Puigdemont of blackmailing the state following reports that the north-eastern region has prepared a law to secede from Spain immediately if it is not allowed to hold an independence referendum.
Mr Rajoy said Mr Puigdemont's alleged plan was "intolerable", labelling it the most serious incident he had seen in his career.
The El Pais newspaper said the Catalan draft law envisages establishing a republic, taking immediate control of the judiciary in the region and seizing state property in Catalonia.
Jordi Turull, a senior member of Mr Puigdemont's governing Together for Yes coalition, denied the report on his Twitter account, saying El Pais' version was out of date and that this would be demonstrated when the law is eventually approved.
The Catalan government has been working on the so-called "disconnection from Spain" bill in secret for several months.
A visibly irritated Mr Rajoy said Mr Puigdemont's plan was to liquidate a state that has been in existence for at least 500 years, and he would not allow it.
He demanded that Mr Puigdemont should present his secession proposal before parliament and explain why he is "threatening and blackmailing the state".
Mr Puidgemont, who has rejected the offer to address parliament, visits Madrid later for a private conference on the referendum.
His government has pledged to hold a vote on secession in September even without clearance by the central government.
Relations between the two governments have soured greatly over the issue in recent years.
Mr Rajoy's government has consistently said an independence referendum is illegal unless Spain's constitution is amended.
Catalonia, whose capital is Barcelona, represents a fifth of Spain's GDP.