MADRID, May 20, 2008 (AFP) - Spain's prime minister on Tuesday once again rejected a Basque government sovereignty plan that calls for a referendum over the future of relations between the region and the rest of the country.

Prime Minister Jose Luis Rodriguez Zapatero said Basque premier Juan Jose Ibarretxe's proposal was unconstitutional and did not have the support of the majority of Basques.

'There will be no act in this country that does not respect democratic rules,' he told a news conference after holding talks in Madrid with Ibarretxe, adding that the proposal was a 'road to nowhere'.

But Zapatero said he would be open to granting more autonomy for the affluent northeastern 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.

Ibarretxe meanwhile said he found Zapatero to be 'defensive' during their talks but said he was still optimistic that an agreement with the central government over his proposal could still be reached.

'The Basque people are persistent and so is the leader of their government,' he said.

Ibarretxe first floated his 'road map' for new political relations for the region with the rest of Spain which last year called for two referendums on self-determination by 2010, after the Spanish government's tentative peace talks with ETA broke down.

The meeting between Zapatero and Ibarretxe came after ETA set off two powerful car bombs over the past week, including one outside of a civil guard barracks in the village of Legutiano which killed one guard.

ETA has killed more than 820 people in its 40-year campaign of bombings and shootings to carve a Basque homeland out of northern Spain and southwestern France.