It is not yet clear if he will declare the restive region's independence from Spain, and whether such a declaration would be purely symbolic. "We will prevent this independence from taking place". This "legal position" establishes the principle according to which a State born out of secession within the EU would not automatically be considered as part of the Union.
France's European affairs minister Nathalie Loiseau said: "If there were to be a declaration of independence, it would be unilateral, and it would not be recognised".
Artur Mas was the Catalan president between 2010 and 2015 and helped negotiate a new Statute of Autonomy with the Spanish Government before it was overruled by the Constitutional Court.
Catalonia accounts for almost a fifth of Spain's economy, and leads all regions in producing 25% of the country's exports, CNNMoney reports.More news: World's best tennis players set to attend 2018 Australian Open
On Monday, Catalan leaders are now facing a huge pressure to drop their plans for independence as France and Germany have expressed their stance that Spain and Catalonia must remain together and remain was one nation.
But Catalan President Carles Puigdemont has announced an important speech for Tuesday evening, in which an independence declaration is likely.
Divisions between Catalan leaders and the central government took a particularly brutal form on the day of the vote, when thousands of Spanish police went to Catalonia to try to shut the referendum down and clashed with protesters and voters.
"In our view, the Catalonian government is not really in full control of the process toward independence, which has been taken over by far-left radical parties and anti-globalization groups".
Tension is building throughout Spain on Monday, with less than 24 hours from a possible declaration of independence from the Catalan government.More news: North Haven Police Wear, Sell Pink Patches for Breast Cancer Awareness Month
At Sunday's rally, demonstrators cheered and applauded when a national police helicopter flew over and some people shook the hands of national police officers to thank them for their efforts to stop the referendum.
Hundreds of thousands of Spaniards rallied Sunday in a tide of red and yellow national flags in what is the worst upheaval since the country returned to democracy in the 1970s.
"There are millions of people who have voted, who want to decide".
"Some of the uncertainty has been reduced - the tone from Puigdemont has become more conciliatory and (Spain's Prime Minister Mariano) Rajoy has also stepped back", said Peter Chatwell, head of euro rates strategy at Mizuho.More news: Reactions to 'Ten Commandments Judge' Roy Moore Winning GOP Primary Runoff