The European Commission has insisted it has no contingency plan to press ahead with admitting new states to the EU if Ireland rejects the Nice Treaty a second time.
A Commission spokesman dismissed as "complete rubbish" a report from Reuters that officials had prepared an alternative enlargement plan that would include elements of Nice in the new member-states' accession treaties.
"There is no contingency plan. If there is a second No, enlargement is in serious trouble," the spokesman said.
Legally it was uncertain if more than five countries could be invited to join on the basis of the Amsterdam Treaty, he said. But, regardless of the legal position, it would be politically impossible to ignore the Irish referendum.
"There would have been two referendums and after two No votes, you'd introduce it by the back door? What kind of political signal is that? You are telling the Irish people to p--s off," he said.
Commission officials were too busy with complicated negotiations with applicant countries to work on any contingency plan, he said.
The success of anti-Nice parties in last week's election and the most recent Irish Times poll on Nice have increased anxiety in Europe that Ireland is likely to reject the treaty a second time.
The Fine Gael MEP, Ms Mary Banotti, said this week she believed the treaty was now doomed, and it would be futile to go ahead with the referendum.
The Commission President, Mr Romano Prodi, said last year enlargement was technically possible without the Nice Treaty, but he said later it would be politically difficult, if not impossible.
The Health Commissioner, Mr David Byrne, and the External Affairs Commissioner, Mr Chris Patten, have both said a second No to Nice would inevitably delay enlargement.
EU leaders agreed at Amsterdam to hold an Inter-Governmental Conference to reform EU institutions before admitting more than five new members.
Some opponents of Nice argue that, by holding the conference, the leaders have fulfilled their own instructions, even if the treaty is not ratified.