"It was a unanimous decision," the conservative prime minister said during a speech.
The motion, passed last month, 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 to completing independence in 18 months.
Rajoy, who is facing a hard-fought battle to retain power in a general election on December 20th, immediately challenged the text in Spain's Constitutional Court, which temporarily suspended it while it considers legal arguments.
The pro-independence camp won an absolute majority in Catalonia's 135-seat regional assembly for the first time in local elections in September but got only 48 percent of the popular vote.
Polls show that most Catalans support a referendum on independence, but are divided over breaking off from Spain.
Catalonia, a northeastern region of 7.5 million people which represents nearly 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.