The future of the European Union was plunged into chaos tonight after Ireland rejected the Lisbon Treaty.
The final referendum result showed 53.4 per cent voted No, while 46.6 per cent voted Yes.
Reacting to the referendum result, European Commission President Jose Manuel Barroso insisted the treaty was "still alive".
But the result, which came despite a vigorous Yes campaign by the Irish government, left EU member states with a major headache.
Eighteen of the EU's members have already ratified the major reform package the treaty introduces. But Ireland's constitution forced it to put the package to a public vote.
Mr Barroso said Europe's governments would now have to decide what to do next but he called on other members states to ratify the treaty.
"I believe the treaty is alive and we should now try to find a solution," he told reporters in Brussels.
He said he had spoken to Irish prime minister Brian Cowen this afternoon, who had insisted that the no vote should not be seen as a vote against the EU.
The verdict represents an embarrassing defeat for Ireland's new premier, who led the Yes campaign which spent millions, but was beaten by a small collection of anti-Treaty groups.
There were 862,415 No votes to 752,451 Yes votes, a majority in 109,964.
Sinn Fein, the only party to oppose the Lisbon accord in the referendum, welcomed the result.
Its president Gerry Adams: "I think that's a very positive thing.
"It was very much a David and Goliath contest ... and in this case Goliath lost again."
"We want to interpret this in a very positive way because it's the people's sign that they want a social Europe.
"They feel secure at the heart of Europe, but they want to ensure there's maximum democratic power."
The result will now dominate next week's EU summit in Brussels where ministers and Commission officials will have to grapple with the question of what to do with a treaty which was supposed to ratified by all member states before it could be introduced.
Mr Barroso said: "The no vote in Ireland has not solved the problems which the Lisbon Treaty is designed to solve.
"The ratification process is made up of 27 national processes. Eighteen member states have already approved the treaty and the European Commission believes that the remaining ratifications should continue to take their course.
"At the European Council (summit), we will want to confer with each other, to hear Prime Minister Cowen's analysis, as well as his ideas on how to address the concerns expressed by those who chose to vote no.
"At the same time, the EU institutions and the member states should continue the work of delivering for the citizens of Europe on issues like growth and jobs, social cohesion, energy security, climate change and fighting inflation.
"Working together in the EU remains the best way to deal with the challenges affecting Europeans today."
The leader of Ireland's second largest party, which backed Mr Cowen's Yes campaign, said they faced the difficult task of recovering from the defeat.
Fine Gael leader Enda Kenny said: "We must move on from here and see how the process can be opened up again so that Ireland can remain as a central player in Europe."
The Government was criticised for failing to adequately translate the complex 287-page Treaty and proved unable to address the No camp claims that the Treaty eroded Ireland's control over its own destiny.
Mr Cowen took over last month from Bertie Ahern as Taoiseach and as leader of the main party of government, Fianna Fail.
His Yes campaign enjoyed the backing of European leaders and was supported at home by all the parties of government and by Ireland's leading opposition parties, Fine Gael and Labour.
The Treaty vote was the first important test of his premiership and he must now attend a summit of the EU leaders in Brussels next week with the difficult task of finding a way forward.
Voters were left confused - and ultimately suspicious - of the Treaty as a piece of complex amending legislation which worked across many areas of government international relations.
It was billed as Europe's attempt to streamline decision-making in the enlarged EU of 27 members states, but voting changes to reflect the population blocs across the EU.