Electoral officials in Ireland said Friday that voters have rejected treaty proposals that would give the European Union a constitution, casting doubt upon future moves toward political integration of Europe.

In a national referendum held on Thursday, voters turned down the Treaty of Lisbon with a national No vote of 53.4 per cent.

The treaty, a blueprint for modernizing the 27-member EU cannot become law without Irish approval.

Rural and working-class voters heavily rejected the treaty to enhance the EU's powers and institutions in line with its rapid expansion since 2004.

Ireland was the only EU member to seek to ratify the Lisbon treaty through a referendum. All others are doing so only through their national parliaments. Eighteen of the 27 member states have already approved the deal.

Irish Prime Minister Brian Cowen will join other EU leaders at a summit next week to try to negotiate a new way forward.

The treaty was negotiated in Lisbon last December by EU leaders as a way of replacing the proposed European constitution, which had earlier been rejected by voters in France and the Netherlands.

It would have provided for many of the same Europe-wide government arrangements foreseen in the rejected constitution, but as an international treaty, only parliamentary approval was necessary for ratification.

The exception was Ireland, the lone European country that requires a referendum on all constitutional changes.

Treaty dead if rejected: French PM

European leaders had been watching the Irish referendum anxiously and have acknowledged that a rejection of the treaty would leave them with no remaining options to give Europe the level of government that they think it needs.

"If the Irish people decide to reject the Treaty of Lisbon, naturally, there will be no Treaty of Lisbon," French Prime Minister François Fillon said Thursday.

Voter turnout was said to be low, around 45 per cent, which analysts said increased the likelihood of rejection of the treaty.

During the campaign, led by Prime Minister Cowen and supported by almost all opposition parties, voters complained that the 300-page treaty was too complicated, and took many national powers away from the Irish government.

Opinion polls across Europe consistently show that political elites in member states are far more enthusiastic about making the EU into a supranational government than ordinary voters.

Irish voters opposed to the Lisbon treaty expressed unease with loss of national sovereignty to the EU. Supporters had said the country had long benefited from membership in European institutions and needed to deepen its involvement in the Continent's political affairs.

The institution that became the EU was founded in the 1950s as a coal and steel free trade zone involving France, Germany and four other countries. Since then it has grown into a 27-nation bloc with an integrated economy larger than that of the United States.