Alfredo Pastor, who served as the Spanish Economy Minister from 1993 to 1995, made the remarks while speaking to BBC Newsnight.
Mr Pastor said: “The referendum was not a referendum to be recognised by anybody.
“The Catalan Parliament had no legal authority to set up such a referendum.
“And the newly declared republic is outside the constitution, which is the instrument we have to frame our democracy.”
The Catalan regional Parliament voted to declare independence from Spain on Friday taking the nation’s worst political crisis in four decades to a new level.
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The declaration of independence by the Catalan Parliament has resulted in investors selling stocks and government bonds after Madrid’s financial market was hit with the news.
The Government in Madrid has refused to recognise Catalonia’s declaration and warned direct rule would be immediately imposed – by force if necessary.
Following the news, the Prime Minister of Spain Mariano Rajoy announced he fired the entire Catalan government, including the region’s president Carles Puigdemont.
Yesterday a top economist warned that Catalonia’s declaration of independence from Spain could be “the foundation for what will happen across the EU”.
Jonathan Davis, an economist with Jonathan Davis Wealth Management, said: “The reason why Catalonia wants to leave Spain is exactly the reason why in Britain we’ve voted for Brexit, why the Americans voted for Trump, because the people are fed up with what governments and socialists are doing to us.”
“Hopefully Catalonia will be the foundation for what will happen right across the EU, governments will see that they’ve got to amend their ways.”
But Mr Davis revealed that despite the UK being the first nation to break from the EU, he did not believe Brexit inspired Catalonia to leave the bloc.