Catalans go to the polls on Sunday morning in a crucial regional election, billed as a de facto referendum on independence, that could launch Spain and the EU into uncharted territory.

Formally the purpose of the vote is to decide the membership of Catalonia’s 135-seat regional government. But after the central government in Madrid shut down attempts by Catalan leaders to hold a referendum on independence, nationalists called early elections that could act as a quasi-referendum on whether the wealthy north-eastern region should seek to break away from Spain.

Pro-independence coalition Junts pel Sí, made up of nationalists from the left and right as well as grassroots activists, have pledged to begin the process of Catalan independence. Far-left Popular Unity, or CUP as the party is known in Spanish, is also committed to unilaterally breaking away from Spain.

Leftist coalition Catalunya Sí que es Pot, backed by parties including ICV and Podemos, have said they favour a referendum on independence.

The regional affiliate of the Socialists have championed a constitutional federal reform which they say will grant Catalonia more powers, while centre-right Ciutadans, the regional arm of Ciudadanos, wants to change the financing system for Spanish regions and more clearly distribute competencies between central and regional governments.

The conservative People’s party, which governs Spain, campaigned in Catalonia on the idea of a unified Spain. On Friday, PP leadership released a video in which the Spanish prime minister, Mariano Rajoy, made a rare declaration in Catalan, arguing: “United we will win.”

Some 5.5 million people in the region are eligible to vote. Polls close at 8pm (7pm BST) and the Guardian will be covering the results as they come in on our live blog.