After Scotland, another European breakaway?
It has been three centuries since Catalonia was independent from Spain, but the day when it regains autonomy seems closer.
Artur Mas, president of Catalonia's Parliament, announced a referendum on the region's independence in November 2014 – which the central Spanish government immediately damned as unconstitutional. Spain's constitution speaks of the "indissoluble unity of the Spanish nation."
Together with Scotland's push for independence, the Catalonian campaign marks a mounting wish by historically autonomous states, which have become provinces, to assert their independence as the euro zone crisis makes their parent states look weaker.
(Read more: Scottish independence: Widnae it work?)
Both places have in common fierce pride in their cultural heritage, a population size which would place them among the European Union's smaller countries, semi-autonomy at the moment, and awareness of their economic importance – Catalonia as a source of exports and Scotland as the part of the U.K. closest to its North Sea oil fields. Agitators for the independence of both regions want to stay within the EU and keep their current currencies.
"Given the economic importance of Catalonia , the degree of uncertainty around the issue could unsettle markets, initially Spanish sovereign bond markets, if not in the run-up to a ballot then in the aftermath of a possible double "yes" vote," Alastair Newton, senior political analyst at Nomura, told CNBC.
(Read more: Why Catalonia should worry markets)
Catalonia accounts for roughly one-fifth of Spain's gross domestic product (GDP), with around 15 percent of its population, and its GDP per capita is the highest in the country. However, Catalonia has been bailed out by the central Spanish liquidity fund to the tune of 14.5 billion euros ($19.9 billion), and it is unclear how the repayment would work if Catalonia is autonomous.
Spain has been one of the euro zone states worst affected by the economic crisis, and had to request a 100 billion euro bailout from international lenders in 2012 to bail out its banks. In the second quarter of 2013, it finally exited recession after more than two years of shrinking growth, but unemployment is still high and its banking sector is still troubled.
(Read more: Spain emerges from recession)
The proposed ballot would put two questions to Catalonians: "Do you think that Catalonia should be a State, yes or no?" and "If yes, do you want that State to be independent, yes or no?"
The European authorities have warned that an independent Scotland or Catalonia would not necessarily be welcomed.
Herman Van Rompuy, President of the European Council, told reporters: "A new independent state would, by the fact of its independence, become a third country with respect to the Union and the treaties would, from the day of its independence, not apply anymore on its territory."
In other words, a new state would not automatically become part of the EU, or indeed the single currency.
(Read more: After Bill Gates, is Spain back in fashion?)
Despite the complications, the Catalonian campaign looks likely to continue.
"It isn't clear at this stage whether it is (a) a gambit to get Madrid to agree to serious talks on fiscal rebalancing, (b) a gauntlet to force a formal ban on a ballot by Madrid with a view to appealing to the European Court of Human Rights (which could take years to reach a conclusion), (c) a genuine and definitive move towards at least an informal indicative ballot, or (d) potentially, all of the above," Newton warned.
- By CNBC's Catherine Boyle. Twitter: @cboylecnbc.