Irish voters go to the polls on Thursday to determine the fate of the European Union's reform treaty in a referendum that could easily go either way.

Ireland is the only one of the 27 EU member states holding a referendum - meaning a country accounting for less than 1 percent of the bloc's 490 million population could derail a pact designed to reform how the bloc is run.

A survey last week put opponents ahead for the first time, causing consternation in Brussels where policy chiefs need ratification from all member states to implement the replacement for a constitution rejected by French and Dutch voters in 2005.

The last opinion poll of the campaign, carried out by Red C and published at the weekend, showed opponents of the Lisbon treaty gaining ground but put the "Yes" vote slightly ahead.

"I would say it is very close but edging towards a 'Yes' depending on who goes out to vote," said Red C Managing Director Richard Colwell. He believes people opposed to the treaty on the grounds they do not understand it may be less likely to vote.

The treaty will create a long-term president of the European Council of EU leaders, a stronger foreign policy chief, a more democratic voting system and give a greater say to national and European parliaments.

Polling stations open at 7 a.m. and close at 10 p.m. No exit polls are planned, adding to the tension among EU policy chiefs who must wait until Friday for the count to begin with a result expected in the afternoon.

Most politicians, businesses, the congress of trade unions and powerful farming groups have called for a "Yes" vote, but concede a complex treaty text has made it a hard sell to voters.

Turnout will be key after Ireland almost scuppered EU plans for eastwards expansion by rejecting the Nice treaty in a 2001 referendum where only 35 percent of the electorate voted.

It was eventually passed in a second vote but Ireland's politicians insist a rerun is not an option this time round. They say the EU has no fallback position given the treaty is already a replacement for the defunct constitution.

Prime Minister Brian Cowen, who has called on people to vote in a matter of "vital national interest", will be relieved by a forecast of fair weather - a factor which could boost turnout.

The "Yes" camp warns Ireland's diplomatic clout and an economy already reeling from a property downturn would suffer if voters were ungrateful enough to reject the reform plans of an EU whose support underpinned the "Celtic Tiger" economic boom.

Opponents such as nationalist party Sinn Fein accuse them of bullying and scaremongering and want voters to reject a treaty that they say should be renegotiated in order to better protect Ireland's sovereignty, military neutrality and influence.