Plans for institutional reform are also on the agenda

But the lavish signing ceremony is expected to be overshadowed by the fallout from the EU's serious divisions about the war in Iraq.

On his way to Athens, UK Prime Minister Tony Blair visited the German Chancellor Gerhard Schroeder - one of Europe's most committed war opponents - in what was seen as a bridge-building mission.

Most EU leaders are thought to be keen to move on from their bitter pre-war splits, and at least agree on how to run and rebuild Iraq after the conflict.

Crucially, they will have to agree on what role the UN should have.

United Nations Secretary General Kofi Annan, also attending the two-day summit, will consult key members of the UN Security Council on their views on a future role for the UN in Iraq.

We must do everything... to reinforce the transatlantic dialogue and avoid any worsening of relations between Europe and the United States Greek Prime Minister Costas Simitis

The last few months have shown just how difficult it will be to turn rhetoric about a unified Europe and a common foreign policy into reality, says BBC Europe correspondent Chris Morris.

UK Prime Minister Tony Blair has called on his European allies to put their differences over the war in Iraq behind them and concentrate instead on finding agreement over the best way to reconstruct the country.

Greek Prime Minister Costas Simitis, whose country holds the rotating EU presidency, also urged European leaders to find some ground for unity.

"We must do everything... to reinforce the transatlantic dialogue and avoid any worsening of relations between Europe and the United States," he said on Tuesday.

"In the long term, nobody can govern the world alone."

But there are still strong disagreements over how Iraq should be reconstructed, both economically and politically.

French President Jacques Chirac and Mr Schroeder have both made clear they expect the UN to play a high-profile, central role, covering humanitarian assistance as well as offering some form of political input.

But the United States, which led the military intervention in Iraq, believes it should decide largely how any post-conflict administration is shaped.

Historic occasion

While Mr Blair is closely allied to the United States on Iraq policy, he is known to be more enthusiastic than many in Washington about a wider role for the UN.

The Athens meeting was called so all 15 leaders of the present member-states could join those from the 10 newcomers, mostly from eastern Europe, in signing the accession treaty.

A spectacular ceremony is to be held beside the Acropolis, a symbol of the world's first democracy.