MADRID, July 15, 2008 (AFP) - Spain's government on Tuesday appealed to the Constitutional Court to block a plan by the northern Basque Country to hold a referendum on self-determination.

The referendum plan, which the Basque parliament approved by a narrow majority on June 27, became law in the region earlier Tuesday when it was published in the official journal.

It envisages the recognition by Madrid of the right of self-determination for the Basque people, a demand of the armed separatist group ETA, blamed for the deaths of 823 people in its 40-year campaign for an independent Basque homeland.

The central government in Madrid has said the referendum is illegal, and had vowed to take the issue before the Constitutional Court on the same day the law is promulgated.

The justice ministry said Tuesday it had filed a legal challenge with the court, saying such a referendum was the 'exclusive competence of the state.'

The head of the Basque government, Juan Jose Ibarretxe, slammed Madrid's 'arbitrary' action, which constitutes 'a direct attack on our autonomy.'

The challenge means the poll will be automatically suspended by the court, which is expected to issue a ruling within five months, the ministry said.

The referendum is the first part of a proposed 'road map' for new political relations between the northern region and Madrid.

Ibarretxe first announced the plan after ETA formally called off a 15-month-old ceasefire in June 2007.

In May, he revealed the questions that would be put to voters on October 25.

1: 'Do you support a process aimed at negotiating an end to violence if ETA first demonstrates, unequivocally, its will to definitively end violence?'

2: 'Do you agree that Basque political parties should begin a process of negotiation to reach an agreement on a democratic accord on the Basque people's exercise of the right of self-determination and that this accord be submitted to a (regional) referendum before the end of 2010?'

Spain's Socialist Prime Minister Jose Luis Rodriguez Zapatero said he would be open to granting more autonomy to the affluent region, which already enjoys a high level of self-government, as long as this respected the constitution and was the result of a broad agreement on the part of Basque political parties.