The enlargement of the European Union was "a profoundly moral issue", the European Commissioner for External Relations, Mr Chris Patten, told journalists in Dublin yesterday.
"It is much the most important thing that will happen during my time as a Commissioner and in Europe in the next decade or more."
Asked about the implications of a second referendum defeat for the Treaty of Nice, Mr Patten said: "You don't have to have an IQ of 140 to understand that there are inevitable consequences."
Opinion surveys showed that there was hardly another country with a more profound sense of its European identity than Ireland.
Asked for his views on the right way to campaign for a Yes vote, he said: "I would try not to be too clever."
But he added: "I would put the arguments pretty bluntly and pretty straight.
"People need a story they can understand and they need a story which is consistent and clear and which stands up to close scrutiny and you always get into trouble in politics where you can't provide that."
The most important argument was a moral one. A No vote meant telling the applicant countries, most of them former victims of totalitarian rule, that they had to go on suffering. It meant saying that, "We don't regard Warsaw and Cracow and Budapest and Prague as European cities in the same way as we do Dublin or Paris or London."
Responding to Mr Patten's comments during his Irish visit, Mr Anthony Coughlan, of the National Platform, said: "Only Ireland's No to Nice stands in the way of the EU being hijacked by the big states, specifically France, Germany and Britain."
Mr Coughlan said opponents of Nice were "defending the concept of the EU as a partnership of legal equals against an effective Franco-German takeover bid. They are defending the rights of the smaller EU member states vis-à-vis the big ones. They are defending the right of the applicant countries to be treated on admission to the EU as political equals of those who are already members of it."
Mr Patten was also criticised by the Green Party MEP, Ms Patricia McKenna, who said: "During his visit here, he was clearly unable to adequately address the concerns of Irish people regarding the Nice Treaty."