Governments in the European Union are exploring what to do after the Ireland's rejection of the Lisbon Treaty to reform the expanded EU.
France and Germany have described the "No" vote as a serious blow but urged the EU to press ahead with the project.
French President Nicolas Sarkozy said ratifications must carry on so that the Irish vote did not "become a crisis".
But Czech President Vaclav Klaus said the treaty was finished, since any further ratification was impossible.
His is a lone voice among EU leaders, but his views will probably resonate with many European voters who did not get a say in a referendum, says the BBC's Oana Lungescu in Brussels.
The third failed referendum on an EU treaty in three years can only be seen as a serious blow to the EU's credibility at home and abroad, our correspondent adds.
'Take their course'
Voters in the Irish Republic rejected the Lisbon treaty in a vote by 53.4% to 46.6%.
The 27-nation EU requires all its members to ratify the treaty but only Ireland has held a public vote.
A referendum was mandatory in the Republic as the country would need to change its constitution to accommodate the treaty.
The European Commission says nations should continue to ratify the treaty, designed to streamline decision-making.
European Commission President Jose Manuel Barroso said that Ireland remained "committed to a strong Europe".
"Ratifications should continue to take their course," he added.
British Foreign Secretary David Miliband said the UK would press on with its ratification.
Lisbon is supposed to replace the European constitution, which was rejected by French and Dutch voters three years ago.
The treaty, which is designed to help the EU cope with its expansion into eastern Europe, provides for a streamlining of the European Commission, the removal of the national veto in more policy areas, a new president of the European Council and a strengthened foreign affairs post.
It is due to come into force on 1 January, 2009.
Fourteen countries out of the 27 have completed ratification so far.
European governments will spend the weekend trying to chart a way forward for the EU, the BBC's Jonny Dymond reports from Dublin.
EU leaders appear to be in for a weekend of deep reflection
Ireland has thrown a spanner deep into the EU's machinery and Europe's leaders have just a few days before they meet for their summer summit in Brussels, to come up with some credible ideas as to how to move forward.
The most obvious course of action might be to tinker with Treaty and then ask Ireland to vote again.
But Thursday's "No" vote was more than the usual anti-European suspects, our correspondent says.
The No campaign successfully increased its vote in the Irish Republic and asking a population to vote again is a trick you can only pull so many times.
The weekend will be a period of deep reflection for many governments across the EU, our correspondent says.