MADRID (Reuters) - Spain's Constitutional Court on Tuesday partially struck down a sovereignty claim approved by lawmakers last year in the northern region of Catalonia, a vital step toward a referendum on full independence, court papers showed.

In January 2013, Catalonia's regional parliament unanimously adopted a declaration of self-determination saying the people had the right to vote on breaking away from the rest of Spain.

The Madrid court declared "null and unconstitutional" the first point in the Catalan ruling which said the people of the region had the legal right to infringe the Spanish constitution.

Catalan President Artur Mas has promised to hold a referendum on secession from Spain on November 9, saying Catalonia should be a separate state within the European Union and the euro zone.

Spanish Prime Minister Mariano Rajoy says such a vote would be unconstitutional.

Mas has been buoyed by a groundswell of public support for the referendum but with the European Union and Spanish government firmly opposed, full independence seems unlikely.

The Catalan Parliament in Barcelona voted in January to send a petition to the national legislature seeking the power to call a popular vote on the region's future. That initiative is still pending a vote in Madrid.

Under the constitution of 1978, Catalonia, worth a fifth of Spain's economic output, has significant autonomy. But perceptions that the taxation system, which spreads wealth between the regions, is unfair have fuelled the rising independence movement.

Mas has suggested that once all legal roads are exhausted, he will use Catalonia's next election in 2016 as a proxy vote on the issue. Opinion polls show more than half of all people from the region support secession from the rest of Spain.

(Reporting by Teresa Medrano, Writing by Paul Day; Editing by Janet Lawrence)