Spain's prime minister has called a referendum on independence for Catalonia "illegal" in his state of the union address.
Mariano Rajoy vowed to block the vote, which the Catalan authorities intend to hold on 9 November.
Both Spain's ruling conservative party and the Socialist opposition have long stated their rejection of a referendum.
But in recent months, the Catalan regional government has vowed to press ahead even without Madrid's blessing.
Mr Rajoy told the Spanish parliament during the annual state of the nation debate that "this referendum can't take place, it is not legal".
He added: "It is the entire Spanish people who have the capacity to decide what Spain is."
Mr Rajoy also addressed Spain's economic problems in his speech, saying that the country had turned a corner and was "part of the driving engine" of Europe.
He also revised the growth forecast upwards from 0.7% to 1% for 2014, and promised tax cuts for 12 million of Spain's 17 million taxpayers.
Tensions between the Spanish government and Catalonia's regional government have been rising in recent months.
Catalan's regional government announced in December that it had decided on the two questions that would be put to the electorate.
Voters would be asked if they wanted Catalonia to be a state and if they wanted it to be an independent state.
However, under current Spanish law, the referendum would not be binding because it has not received the backing of the central government.
Last week the Spanish parliament voted overwhelmingly for a motion rejecting the referendum.
Catalan President Artur Mas said this month that the referendum could be better thought of as a "consultation".
Catalonia is one of Spain's most developed regions, with a population of 7.5 million.
It already has a wide degree of autonomy but the recent economic crisis has fuelled Catalan nationalism.
Opinion polls suggest Catalans are evenly split over independence.
The EU and Nato have warned that Catalonia would be excluded if it broke away from Spain.
Nationalists in another Spanish region, the Basque Country, won regional elections there last year.