Spain's Constitutional Court ruled Tuesday that a referendum law passed in Catalonia, which paved the way for a contested October 1 secession referendum, was not valid.
The court temporarily suspended the law after it was passed by Catalonia's regional parliament on September 6 while judges considered an appeal against it filed by Spain's central government.
Catalonia's pro-independence regional government went ahead with the referendum on October 1 despite the suspension and warnings from Madrid that the vote was illegal.
The court said in a statement Tuesday that its 12 judges had "unanimously" declared the referendum law unconstitutional.
"The 'right to self-determination' does not exist for any of the 'peoples of Spain'," it said, adding that the "right" to "promote and enact the unilateral secession" of a part of the country is not recognised in the Spanish constitution.
Since 2014 Spain's courts have systematically struck down decisions taken by the Catalan government and parliament regarding holding an independence referendum.
Madrid has given Catalan leader Carles Puigdemont until Thursday to clarify whether he is declaring independence following the referendum, which resulted in a 90 percent 'Yes' vote -- although turnout was only 43 percent as many supporters of Spanish unity stayed away.
Puigdemont stopped short of giving the definitive response that Madrid had demanded on Monday and instead repeated his call for talks with Spanish Prime Minister Mariano Rajoy.
But anything less than a full climb-down is likely to prompt the central government to start imposing unprecedented direct control over the semi-autonomous region -- the so-called "nuclear option".