Spain's government has vowed to block the region of Catalonia's plan to hold an independence poll on November 9 next year.
"The poll will not be held," Justice Minister Alberto Ruiz-Gallardon told journalists on Thursday, just moments after Catalonia's President Artur Mas announced a deal among regional parties for the date and wording of a referendum.
Ruiz-Gallardon's reaction was later echoed by Spain's Prime Minister Mariano Rajoy, who vowed to block the independence poll.
"As prime minister I have sworn to uphold the constitution and the law and, because of this, I guarantee that this referendum will not happen," Rajoy said during a joint news conference with European Council President Herman Van Rompuy.
"Any discussion or debate on this is out of the question."
Mas announced in the Catalan capital, Barcelona, that the referendum would ask the region's voters if they want Catalonia to be a state and, if so, should it be independent.
Mas did not clarify the distinction between a state and an independent state. However, the questions appeared to open a door for those nationalists who want Catalonia to have the structure of a state but remain a part of Spain, possibly along the lines of Puerto Rico and the United States.
But Ruiz-Gallardon said the referendum would be illegal and would not be allowed.
Spain's constitution says only the central government in Madrid can call a referendum. The government has not said what it might do to prevent a ballot.
Mas said the referendum date was set almost a year away so as to give ample time for negotiations with Madrid on "the way to stage the consultation legally".
Polls indicate that Catalans are roughly evenly split on the issue of independence. The European Union and NATO have warned Catalonia it would be excluded if it seceded.
Scotland is staging an independence referendum next year, on September 18. That vote has been approved by the UK government.
Mas began pushing for a referendum after he failed to clinch a better financial pact for Catalonia with the central government in 2012. The referendum proposal got the support of some one million people who turned out at two demonstrations held since then.
The possibility of a region having the right to decide its future has stirred much political debate and raised questions as to whether it is time to reform the 1978 Constitution to ease territorial discontent.
The Basque region, which has traditionally sought greater powers, failed in a bid to hold a self-determination referendum several years ago.
Catalonia is one of the country's most powerful regions and represents roughly a fifth of Spain's $1.5 trillion GDP. Its population of 7.5 million is greater than those of EU members such as Denmark, Ireland or Finland.
The region, like others in Spain, has its own language in addition to Spanish. Its financial powers include some tax collecting rights.
Spain has 17 regions, each with substantial autonomy but with no control over key areas such as defense, foreign affairs, ports and airports, and in the making of national economic and financial decisions.