Spanish Foreign Minister Miguel Angel Moratinos on Saturday urged his country's voters to turn out in force for a referendum on the EU constitution scheduled for February 20.

Spain is the first of around a dozen states in the 25-member bloc who will submit the text to a popular vote with the remainder leaving ratification to parliament, and Moratinos said a strong turnout was vital.

"The most important thing is to go out and vote," said Moratinos, who added that "massive participation" would underpin the legitimacy of what is nonetheless a consultative exercise as the final word lies with the Spanish parliament.

The Socialist government strongly backs the constitution as vital to setting in place a decision-making framework for the Union, which welcomed 10 mostly Eastern European members last May.

Spanish opinion polls point to a clear win for the 'yes' camp, although most voters profess general ignorance of the text itself.

Moratinos urged those who were sceptical to take part in the process.

"People must go out and use their chance to express their criticisms, questions or doubts," he told a civic forum in the southern city of Cordoba, insisting a vote in favour would bring the EU closer to its citizens.

"The citizen has to be the protagonist of European construction," Moratinos said.