Sweden, Finland and Portugal have joined other EU members in postponing votes on the ill-fated constitution.
EU leaders agree a wider public debate is needed
EU leaders agreed on Thursday to extend the November 2006 deadline to ratify the treaty, which was resoundingly rejected by French and Dutch voters.
EU President Jean-Claude Juncker, chairing the Brussels summit, said a "period of reflection" was necessary.
Earlier Denmark, Ireland and the Czech Republic said they were postponing their referendums.
Nine states have ratified the treaty - only Spain so far through a referendum.
Portugal has now postponed a referendum it was due to hold in October. In Sweden and Finland the treaty was to be put before parliament for ratification.
European Commission President Jose Manuel Barroso said it was important to consider why French and Dutch voters had rejected the constitution, but it was too early to abandon the treaty.
"We don't want to give up the constitution, nor do we want to carry on with business as usual and pretend nothing has happened," he said.
"We have decided to undertake a stock-taking exercise and have a wide-ranging debate amongst all Europeans. We can't, at the moment, forecast the results of that debate."
Swedish Prime Minister Goran Persson said his country would not ratify the treaty unless France and the Netherlands held new referendums on it.
"If they are not ready to go to their people again... well, then it has fallen by definition," he said.
French President Jacques Chirac has suggested holding an emergency summit to discuss the EU's future.
"In this new situation, can the union continue to expand without us having the institutions capable of making this enlarged union work efficiently?" Mr Chirac said.
Mr Juncker, Prime Minister of Luxembourg, which holds the EU's rotating presidency, insisted there would be no renegotiation of the constitution, which needs to be ratified by all 25 member states.
"We believe that the constitutional treaty has the answers to many questions that Europeans are asking, so we believe the ratification process must continue," he said.
Unlike other EU members, Estonian Prime Minister Andrus Ansip said his country would press ahead with ratification as it was a "good treaty for Europe and also a good treaty for Estonia".
Mr Juncker said most EU leaders agreed that the bloc should honour its pledges to take in more new members.
Romania and Bulgaria signed accession treaties in April paving the way for them to join in January 2007, although a one-year delay is still possible.
Croatia's membership talks have been delayed because of a row over a fugitive war crimes suspect, General Ante Gotovina.
Turkey is due to begin accession talks on 3 October, although politicians in several EU countries have voiced opposition to Turkey joining.