Jean-Claude Piris said it would be “impossible” for an independent Catalonia to join the EU amid an ongoing row over its future as Spain is plunged into its worst constitutional crisis in decades.
Mr Piris, who was Legal Counsel of the European Council for 22 years, says a declaration of Catalan independence would likely “violate the rule of law”.
He said: “It is dangerous to create false illusions about unilateral independence.”
Mr Piris said that despite 90% of voters in favour, Catalonia would not be recognised as a state and would therefore be ineligible for EU membership.
He said Catalonia did not have the right to self-determination.
Mr Piris said: “This right only applies to a territory when there is no democracy in the state from which it wants to separate, or it oppresses minorities, or the territory constitutes a colony.”
Catalan independence would mean the region could not remain in the EU and instead would have to apply to become a member through the normal application process which can take years.
Mr Piris said even if Catalonia had respected Spanish law it could not legally stay part of the bloc. He said: “They would have to follow the normal procedure: request to be a member.”
Asked by Spanish newspaper El Pais about the impact of independence on Europe, he said: “The outcome for the EU can only be negative if an independent Catalonia joins the EU.
“European institutions would be weaker: there would be even more imbalance towards small states. And no citizen would gain new rights.
“I do not see any positive result from the European point of view.”
Mr Piris added the European project was based on solidarity, but Catalonia was clearly not willing to show solidarity with Spain.
He also addressed police violence in Catalonia by the Spanish authorities, saying the clashes did not constitute a human rights violation.
Images of Spanish police and civil guards brutally dragging voters away as they tried to shut down polling stations shocked the world.
But Mr Piris said there was “no basis” for claiming this was a human rights violation.
He asked: “Can we say that Spain is an undemocratic country that crushes minorities?”
He added the EU would not intervene in Spain, a democratic country, and that a resolution would have to be reached between Spain and Catalonia.
It comes as Catalan authorities declared more than 90 per cent of voters were in favour of independence.
(Additional reporting by Maria Ortega.)