The European Union's plans to admit up to 10 new countries could go off the rails if Irish voters once again throw out the treaty that is at the heart of the process.
The EU's enlargement commissioner, Mr Guenter Verheugen, said another No vote in Ireland's second referendum on the issue - to be held on October 19th - would be interpreted as closing the door on eastern Europe.
Mr Verheugen, said he hoped Ireland would ratify the Treaty of Nice in a referendum, after rejecting it last year. Mr Verheugen also reminded the Irish that - as he put it - EU membership had transformed their country from one of Europe's poorest into one of the continent's most developed.
This is among the strongest warning yet that has been given to Ireland not to jeopardise enlargement.
Asked by journalists whether enlargement itself could be seriously delayed by another rejection, the commissioner replied: "It can, yes."
The Taoiseach Mr Ahern is putting the treaty to the State's three million voters for a second and last time in a bid to save face and keep the wheels on the enlargement bandwagon.
The October vote falls only days before an EU leaders' summit in Brussels, which will prepare for the expected announcement in Copenhagen in December that up to 10 eastern European and Mediterranean nations will join the bloc from 2004.
Mr Verheugen reminded the Republic of Ireland that its membership of the EU had helped it to transform itself from one of Europe's least developed economies into one of the strongest since it joined the bloc in 1973.
"So I think that people in Central and Eastern Europe would have difficulties in understanding a rejection in Ireland," he said. The EU's applicant countries "would understand this as the closing of a door".
Ratification of the Nice Treaty was an essential precondition for enlargement to happen without delay, Mr Verheugen said. "Let's pray and let's hope that the Irish will make a wise decision."