A second rejection of the Nice Treaty could have disastrous consequences for the Irish economy, the Taoiseach last night warned the employers' organisation IBEC.
Failure to ratify Nice risked undermining the Republic's standing in the EU, making it less attractive to foreign investors, Mr Ahern said. In such circumstances, the State was in danger of becoming a semi-detached member of the EU.
He cited a recent report from the Economist Intelligence Unit, which warned that a rejection of the treaty would cast doubt on Ireland's commitment to Europe and fatally damage relations with countries applying for EU membership.
It would also deter multinational corporations from investing here and hamper exports.
The Taoiseach was addressing IBEC's biennial president's dinner at the Burlington Hotel, Dublin.
He said: "If Ireland fails to ratify Nice before the end of this year, the European Union will face an unprecedented and unpredictable crisis.This is not in Ireland's interests."
Repeating his promise to seek a declaration from the European Council confirming Ireland's policy of neutrality at today's summit of EU leaders in Spain, Mr Ahern urged business leaders to support his campaign in favour of the ratification of Nice.
The Government will hold a second referendum on the issue in August.
"The Treaty of Nice is essentially for the European Union to enlarge. That is the position of the member-states and of the institutions. It is also critically the position of the countries awaiting agreement on their entry to the Union. Enlargement is the single most important challenge facing the Union."
He added: "It is in our interests to enable enlargement. It is not in our interests to block it."
"I believe passionately that the European Union is vital for Ireland's future prosperity and well being. It is vital for the prosperity and well-being of all the peoples of Europe."
Seeking to appeal to the audience's business instincts, Mr Ahern likened the ratification of the treaty to the revision of a company's articles of association ahead of a key expansion. He said: "You will understand that any company must revise its 'memo and articles' when it enlarges with new component parts, so that it can work efficiently and effectively.
"So, at Nice, the European Union made the adjustments necessary for enlargement. Ireland participated fully in the negotiations that led to the Treaty of Nice. That treaty fully protects our interests."
He added: "That is why in our election manifestos and in the Programme for Government, both Government parties reconfirmed our commitment to hold another referendum later this year."
The Taoiseach also called on IBEC to support a renegotiation of the Programme for Prosperity and Fairness, arguing that it was the best way to safeguard growth.