resolution declares that the parliament of Catalonia is "sovereign" and not subject to decisions made by Spanish institutions, including the Constitutional Court.
Catalonia, a region of 7.5 million people with its own language, that accounts for a fifth of Spain’s economic output, already enjoys a large degree of autonomy in education, health and policing.
But it is insisting on even greater autonomy, particularly where taxation is concerned, estimating that it gives more to the central government than it receives.
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.
Rajoy met earlier on Wednesday with the head of the main opposition Socialists, Pedro Sanchez, who reiterated that he backed the prime minister in his defence of "the constitution and legality".
The challenge laid down by Catalan separatists was "extremely serious", Sanchez added during an interview with private television Telecinco.
But at the same time he accused Rajoy’s government of "inaction" in the face of the separatist challenge and reiterated his call for a reform of the constitution to make Spain a federal state as a way to meet Catalonia’s demands for more autonomy.
While the government prepared its lawsuit, public prosecutors released an 11-page report that said the promoters of the Catalan resolution could face criminal charges of "sedition" and "disobedience".
The report said that those found guilty of sedition could be face a jail term of up to 15 years.
The Constitutional Court will begin examining the government’s appeal of the resolution at 4:30 pm (1530 GMT), a judicial source said.
Catalonia tried to hold an official referendum on independence last year, but the Constitutional Court ruled it was against the constitution, arguing that all Spanish people have the right to decide on matters of sovereignty.