A small, left-wing party on Sunday refused to support Catalan separatist Prime Minister Artur Mas, putting in doubt the independence of the north-eastern Spanish region.
Members of the Popular Unity Candidacy (CUP) party voted in Barcelona against backing Mas as the parliament leader.
More than three months after the September 27 elections, the formation of a new government in the economically strong but politically unstable region has, in all likelihood, permanently failed.
Mas' alliance, Together for Yes (Junts pel Si), has been struggling to stay in power as a minority government since the elections.
While the alliance won the most seats, it failed to reach an absolute majority. To continue to govern, he was dependent on the support of the left-wing party.
While Mas took over as acting premier following the September vote, fresh elections now seem likely.
The anti-capitalist CUP had promised its supporters during the campaign not to back the re-election of the liberal government.
The deadline for forming a government expires January 10.
Catalonia has a population of 7.5 million. Its economy constitutes one-fifth of Spain's gross domestic product. It also has its own culture and language.