The fate of the European Union's first constitution hung in the balance at a decisive summit tonight as leaders appeared deadlocked over the vexed question of voting power within the bloc.
Italy, which as EU president has struggled since October to steer 25 present and future member states to agreement on the historic treaty, prepared the way for possible failure saying it would not accept a deal at any cost.
Italian Prime Minister Mr Silvio Berlusconi, who is leading the talks, raced through the regular summit agenda in the first session today to free up more time to focus on the new charter, designed to streamline EU institutions.
"The voting system is the obstacle that can block the whole agreement, and that is a pity," Mr Berlusconi told reporters.
The Taoiseach has warned that Ireland faces a major task at the meeting in Brussels in retaining concessions it has received on justice and defence issues and in retaining the national veto on all taxation matters.
Mr Ahern said he was content with the current proposals on defence and other key issues for Ireland but he warned pre-summit deals on these issues could unravel over the next two days
Minister for Europe Mr Dick Roche said he "fervently hoped" there would be an agreement on voting rights and the issue would not spill over into the Irish presidency.
"We have a lot of ambitions for our presidency. I think a lot of people are depending on the Irish presidency to pick up a lot of loose ends that are around and to make sure the enlargement process goes ahead," he said.
The core of the constitution debate is how much power the four biggest states - Germany, France, Britain and Italy - will wield and how much integration EU leaders can swallow.
The row pitted Germany and France against two of the continent's medium-sized states, Spain and Poland, that are determined to cling on to existing voting rights out of proportion with their populations.
A morning meeting between British Prime Minister Mr Tony Blair, French President Jacques Chirac and German Chancellor Gerhard Schroeder brought no breakthrough, one diplomat said.
"The positions are a long, long way apart," Mr Blair told reporters. "It is important to try and get an agreement. It may well not be possible."
Italy hopes Mr Blair, who has close ties to the Spanish and Polish leaders, might help to pressure the two countries to compromise in return for acceptance of Britain's "red lines" of keeping a national veto on tax, foreign and justice policy. Mr Chirac stuck to his guns at a news conference, saying France would not accept a "warped result".
The aim of the constitution treaty is to simplify EU decision-making and give the bloc more say on the world stage by creating the position of an EU foreign minister and strengthening the post of EU president.
Diplomats say failure could paralyse the EU as it prepares to expand to 25 members in May with the inclusion of Poland and seven other ex-communist eastern states plus Cyprus and Malta.
Earlier, the leaders adopted a security strategy designed to make the EU a more effective actor in world affairs and prevent divisions of the sort it suffered over the US-led war on Iraq.
They also approved complex arrangements by its main powers, France, Germany and Britain, on a military planning cell for crisis management operations, which was watered down during weeks of negotiation amid US suspicions of its impact on NATO.