"If a part of a member State (of the EU) ceases to be part of that State because it becomes independent, (EU) treaties stop applying to that territory," said EU spokesperson Alejandro Ullurzun on Wednesday.
"In other words, a new, independent State would become, as a result of it becoming independent, a third party in terms of the EU and its Treaties," he added, echoing the official position of the EU on a possible independent Catalonia.
Ullurzun's statement came a day after a pro-Catalan independence pressure group released a report arguing Catalonia would remain in Spain if it left the EU.
A "pragmatic" and "flexible" EU would end up accepting an independent Catalonia as a member state on political and economic grounds, the National Transition Advisory Council said in the document.
That report came hot on the heels of an April 9th vote in the Spanish Parliament rejecting a petition by the Catalan Government to be allowed to hold a referendum on the issue of independence on November 9th.
But regional president Artur Mas, has said that rejection is not final and the region will continue to seek legal means in its fight to independence.
Proud of their distinct language and culture, a growing number of Catalonia's 7.5 million residents feel the redistribution of their taxes to other regions is unfair and that the region would be better off on its own.
In 2013, the Catalan parties Convergence and Union (CiU) and the ERC launched an independence bid with Mas saying a vote on the issue of self-rule would be held in 2014.
The referendum would ask voters two questions:
– "Do you think that Catalonia should be a State, yes or no?"
– "If yes, do you want that State to be independent, yes or no.
Catalonia argues that a 2006 Catalan autonomy statute passed by Spain's parliament granted the region the power to hold referendums.
The April 9th vote in the Spanish Parliament was, however, the latest rejection by Madrid of this argument.