Lawmakers in the regional assembly are expected to approve a resolution that kicks off the 18-month process towards independence from Spain, in what will be yet another major headache for Prime Minister Mariano Rajoy ahead of general elections next month.
The resolution has the backing of Catalan president Artur Mas' Together for Yes coalition and the smaller far-left separatist CUP party, which together have a majority in the regional assembly with 72 seats of the 135 seats.
But Spain's conservative central government has vowed to go straight to the Constitutional Court to declare the resolution void if it is passed.
'Construction of a state'
If the court accepts the government's appeal as is expected, the Catalan resolution will be automatically suspended until judges hear arguments and make their decision.
But those who crafted the declaration had anticipated this, and the resolution states that the secession process will not be subject to decisions made by Spanish institutions, including the court.
While not legally binding, the text calls on the regional assembly to start working on legislation within 30 days to create a separate social security system and treasury, with a view of complete independence as early as 2017.
"With this resolution, we are solemnly kicking off the construction of a state," Raul Romeva of Together for Yes told regional lawmakers.
Rajoy, who is gearing up for a December 20 general election, has the backing of the main opposition Socialists and new centre-right party Ciudadanos, which was born in Catalonia where it enjoys a lot of support.
Albert Rivera, the youthful head of Ciudadanos, warned again secession in a Monday radio interview.
"To those Catalans who want independence: the solution is not to break up the country, it is to reform it," he said.
Among Spain's main parties at the national level, only far-left Podemos has resisted Rajoy's effort to forge a united front on the issue.
While Podemos wants Catalonia to stay within Spain, it has also said it would support a referendum on the matter.
'Illegal and undemocratic'
Rajoy's government in September boosted the powers of the Constitutional Court to allow it to quickly suspend leaders who disobey its orders, in a move aimed directly at Catalonia.
The government has also raised the possibility of invoking article 155 of the Spanish constitution, which allows Madrid to supersede the authority of a regional government that is acting outside the law or cut off its funding.
There have long been demands for greater autonomy in Catalonia, a region of 7.5 million people with its own language that accounts for a fifth of Spain's economic output.
These calls have intensified in recent years, in tandem with the country's economic crisis.
A 2010 decision by Spain's constitutional court to water down a 2006 statute giving the region more powers has added to the growing pressure for secession.
Catalonia tried to hold an official referendum on independence last year, but judges ruled it was against the constitution, arguing that all Spanish people have the right to decide on matters of sovereignty.
The region pressed ahead anyway and held the referendum, although it was purely symbolic in nature.
Turnout was just 37 percent, of which over 80 percent voted in favour of independence.
"A referendum would be the ideal tool but the Spanish government blocked
it. We have no other option but unilateral action," CUP lawmaker Albert Botran told news agency AFP.
While Catalan separatist parties won a majority of seats in the regional parliament in September polls, they failed to win a majority of all votes cast – a fact emphasized by their opponents.
"Everything is unconstitutional, illegal and undemocratic, because they lie when they say the majority of Catalans back a decoupling," said Rivera.