Portugal's President Jorge Sampaio will campaign for a "yes" vote in a referendum on the first ever European Union constitution to be held in the country at the end of the year, he told AFP in an interview.

"I cannot conceive of Portugal on the sidelines of the European Union and I think the president has the obligation to say he will vote 'yes' and why he will vote 'yes'," he said late Wednesday.

"I am in favour of the treaty because I am in favour of an instrument which will allow us to advance," he added.

"It certainly falls short of what some would have liked, but it will allow us to advance in the institutional, economic and social construction of the bloc, in some cases timidly, in others a bit better."

Sampaio was speaking to AFP ahead of an official state visit to France which he will carry out between April 11 and 14.

New Portuguese Prime Minister Jose Socrates proposed holding a planned vote on the treaty, which is meant to streamline decision-making in the expanding bloc, at the same time as municipal elections which are expected in October.

Portugal's parliament was to take steps later Thursday to change the nation's constitution to allow a referendum to be held at the same time as an election for public office.

The EU constitution must be ratified by all 25 member states before it comes into force.

About half of the member states will hold a popular vote on the treaty while the others will ratify it in parliament.

France will vote on the treaty on May 29, making it only the second member state to hold a referendum on the constitution after Spain, which approved it by a wide margin in a vote marked by low turnout on February 20.

French President Jacques Chirac has come under fire for not doing enough to promote a "yes" vote as polls published in recent weeks have shown a majority of voters in France plan to vote against the treaty.