Catalan president Artur Mas signed a decree on Monday night setting the date for the vote in the wealthy northeastern region, home to 7.5 million people and accounting for a fifth of Spain's output.
The decree made no reference to independence for Catalonia, but Mas -- who already faces criminal charges for staging a mock independence referendum in November despite Madrid's objections -- has said that if an alliance of pro-secession parties wins a majority, they will aim to split from Spain within 18 months.
"Politically they will not be normal elections, politically they are a plebiscite on Catalan freedom and sovereignty," Mas said Thursday.
The regional government has already started setting up institutions of state, which would swing into gear if the pro-independence camp wins.
"We are ready," he has repeatedly said during public appearances.
Pro-independence Catalans hold up the Catalan flag during a rally. Photo: Josep Lago / AFP
Prime Minister Mariano Rajoy took to Twitter to dismiss of the Catalan threat insisting that "No-one is going to break up Spain nor transform the Catalans into foreigners within their own country. This government will not allow it."
Nadie va a disolver #España ni va a convertir a los catalanes en extranjeros en su propio país. Este Gobierno no lo va a permitir— Mariano Rajoy Brey (@marianorajoy) August 4, 2015
Last week, Catalan officials presented plans for a future Catalan tax agency and adopted a decree paving the way for a public credit institution to be turned into a Catalan central bank.
The issue of Catalan independence had fallen off the headlines in recent months, with Prime Minister Mariano Rajoy's government more focused on the challenge in a year-end general election from new anti-austerity party Podemos than from Catalan separatists.
But it bounced back into the political spotlight in July after Catalan pro-independence parties set aside their differences and agreed to run on a joint ticket in the regional polls.
The joint list, called "Together for Yes", includes the ruling CDC party, the left-wing separatist ERC and associations which have organised massive pro-independence demonstrations in the region.
A smaller far-left party supporting independence, CUP, has refused to join
the alliance but could prove key to achieving the majority in the regional assembly which the pro-independence camp says would be sufficient to implement its secession plans.
Spain's central government has vowed to oppose Catalonia's moves. "There will be no independence for Catalonia," Rajoy told a news conference on Friday.
During a visit to Catalonia last month, King Felipe VI warned Catalan leaders that "respect for the law is the source of legitimacy and an unavoidable requirement for living together democratically in peace and
Several members of Rajoy's cabinet have raised the possibility of invoking article 155 of the constitution, which allows Madrid to supersede the authority of a regional government that is acting outside the law.
The separatists have warned that if the government uses this article, they will declare unilateral independence.
The movement appeared to lose steam after the Catalan regional government staged a symbolic independence referendum on November 9, despite having been blocked by the courts from going ahead with it.
Polls showed that for the first time since 2011 support for Catalonia remaining a part of Spain surpassed support for independence.
But the pro-independence list has turned the situation around and its opponents are clearly worried.
Catalonia's three pro-independence parties could capture 68-72 seats in the region's 135-seat parliament, according to a poll published last month in Catalan daily La Vanguardia.
The vice president of the Catalan government, Neus Munte, told Spanish public radio on Monday that the pro-secession camp expected to win "a sufficient and solid" majority in favour of independence.
But she refused to specify what this would mean in terms of number of votes or seats won.
Deputy Prime Minister Soraya Saenz de Santamaria has said, however, that under the constitution the election could only be about choosing a new Catalan parliament.
"This is what citizens are deciding with this vote and nothing else," she told a news conference on Monday.
Albert Rivera, leader of the centre-right party Ciudadanos, which is staunchly opposed to Catalan separatism, has said a victory for the pro-unity camp was vital "in order to create a new majority".
Such a majority would require an unlikely alliance between the Popular Party, Ciudadanos, the Socialists and the Christian Democrat party UDC, which in June ended a 37-year alliance with Mas's party due to differences over the independence issue. It would also require the backing of a left-wing platform
set up by Podemos.
"There is a social majority in favour of unity, but it is not mobilised and lacks cohesion. The separatists could win," said Nacho Martin Blanco, a political analyst and commentator with conservative daily ABC.