Spain's Government says it will oppose in the constitutional court a plan for greater autonomy in the Basque region.
The plan will give extra powers to the region
It was responding to a proposal for a referendum in the Basque region on changing its relationship with the rest of Spain to one of "free association".
Justice Minister Jose Maria Michavila said the plan was a deliberate attempt to break with Spain's constitution.
The Basque government has approved the proposal's text and the parliament is due to debate it next week.
Spanish Prime Minister Jose Maria Aznar says the proposal legitimises what he called the terrorism of the armed separatist group ETA.
On Monday he denounced the plan as "a secession plan hung on the 1,000 dead bodies that were lost to terrorism in Spain in recent years".
The plan envisages extra powers for the region, including separate courts and representation in such organisations as the European Union.
It will allow Basques the right to determine their sovereignty by referendum.
Basque prime minister Juan Jose Ibarretxe says the plan will help to end more than 30 years of violence by ETA.
He hopes that the regional parliament will vote on the plan late next year, paving the way for a referendum in 2005 at the earliest.
But Mr Michavila says the plan violates the constitution in more than 100 places, including in areas relating to the division of power between Madrid and Spain's 17 semi-autonomous regions.
"In a tenacious, conscious and deliberate way, the plan aims to break with and unilaterally mutilate the constitution," he said.
Mariano Rajoy, secretary-general of the governing Popular Party, has described the plan as treason.
All Spain's other main central parties are opposed to the plan.