Irish voters had their say over the European Union's Lisbon treaty in a referendum on Thursday, the only citizens in the 27-member bloc to be entrusted with a vote on the replacement for a rejected EU constitution.

The treaty, intended to make the EU stronger and more effective, has the backing of all the main political parties in a country that has prospered from its membership of the bloc.

Bookmakers say the odds are heavily in favour of a "Yes" vote but an opinion poll last week had put opponents of the treaty briefly ahead, alarming EU leaders.

The entire project could be doomed if Irish voters reject it and no alternative plan has been prepared. The outcome will not be known until Friday.

Public broadcaster RTE said turnout appeared light, hovering close to 20 percent in most areas by late afternoon although polls do not close until 10 p.m. Light turnout could help the "No" camp whose backers have strongly held views.

"I genuinely think a lot of people will not make up their mind until they stand in the ballot box with a pen in their hand," said Damian Loscher, head of pollsters TNS mrbi.

The last opinion poll of the campaign, published at the weekend, showed the "Yes" vote narrowly ahead.

The approval of all member states is required to ratify the treaty which replaces a constitution abandoned after it was rejected by French and Dutch voters in 2005.

This time all other EU countries have avoided holding popular votes. Ireland's constitution requires a referendum on any amendments, giving make-or-break power to voters in a nation with less than 1 percent of the EU's 490 million population.

The treaty envisages a long-term president of the European Council of EU leaders, a stronger foreign policy chief and a mutual defence pact. It would also develop a more democratic voting system and give a greater say to Europe's parliaments.

Finance Minister Brian Lenihan told Reuters canvassing by his party indicated the "Yes" camp had regained momentum and said the EU had no fallback position if the pact is rejected.

"You are talking about a coat that was knitted together by 27 different countries," Lenihan said after casting his vote in Dublin. "It is very difficult to knit that coat again."

Fourteen countries have already ratified the treaty in their national parliaments. The treaty is due to come into force on January 1 if all nations ratify it.

EU leaders fear some countries such as Britain may suspend the process if Ireland votes "No". A senior EU diplomat said Britain had told its EU allies it had "no intention to pull the plug on this" even if the Irish vote no.

The "Yes" camp says Ireland's diplomatic clout and economy would suffer if voters rejected reforms drawn up by a union whose support underpinned the "Celtic Tiger" economic boom.

ECONOMIC CHALLENGES

Data on Thursday showed shoppers reining in spending. Unemployment is rising, although below the European average.

"We face economic challenges and Europe provides a framework of stability for investment in Ireland. If you create uncertainty in that framework, that is bad for Ireland and bad for Europe," said Lenihan.

A "No" vote could dent the image of a pro-European, English-speaking euro zone country, although economists do not predict grave long-term consequences.

Voting before heading into work, 45-year-old John Devlin said he expected the vote to be "very tight" but he hoped and believed it would be carried.

"Ireland was a dusty backwater until we joined the European Union," Devlin said. "I think in Europe we should be united and seen as a counterbalance in global affairs."

Opponents such as nationalist party Sinn Fein say Brussels and the government have tried to bully people into backing the treaty and say it should be renegotiated to better protect Ireland's sovereignty, military neutrality and influence.

"The "Yes" campaign believe they have this in the bag," said Mary Lou McDonald, a Sinn Fein member of the EU parliament. "It will depend on turnout. I think it will be a close call."

Low turnout was a factor when 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. That was passed in a second vote criticised as undemocratic.