Spain's Socialist Prime Minister Pedro Sanchez has favoured dialogue with Catalonia to rebuild relations after a chaotic unilateral bid for independence in 2017 plunged Spain into its worst political crisis in years.
It remains, however, staunchly opposed to independence and has hitherto ruled out a legal referendum.
A similar proposal by Catalonia in 2012 was firmly rejected by the then conservative government in Madrid.
The wealthy northeastern region held a referendum five years later despite a ban by the courts, and issued a short-lived unilateral independence declaration.
Catalan government head Pere Aragones told the regional parliament that for another referendum, Catalonia needed Madrid's buy-in.