The Taoiseach will join other European Union leaders in Brussels for a summit focusing on negotiating a new constitution and EU enlargement.
After weeks of sometimes bitter public skirmishing, leaders of the 15 member states and 10 joining countries confront the most sensitive issues of voting rights, seats on the executive European Commission and EU defence integration.
The disputes pit a majority of small member states, fearful of losing influence, against the big EU powers - Germany, France, Italy and Britain - determined that their population size and political weight be reflected in EU decision-making.
Poland, determined not to be undermined by the bloc's giants when it joins the EU next year, has joined hands with Spain to oppose any change to countries' weighted votes that were sealed by the treaty of Nice at the end of 2000.
Final decisions are not expected until December, but European Commission President Mr Romano Prodi warned leaders yesterday against plunging Europe into an unseemly power struggle in the coming weeks through "narrow nationalism".
He also urged the EU's Italian presidency to put forward compromise proposals soon since time is running short.
France and Germany, keen to see a draft constitution drawn up by a Convention of lawmakers and national delegates in June adopted with as few changes as possible, will take the symbolic step of speaking with a single voice at tomorrow's session.