All member states have in principle accepted new "double-majority" voting rules, under which decisions are taken by a majority of member states representing a majority of the EU population. This system would apply to all EU decisions that are not subject to national vetoes -- which constitute a majority.
However, the precise thresholds remain a matter of intense dispute. The Irish EU Presidency last night issued a proposal under which a decision is taken if it is supported by 55 percent of the member states totaling 65 percent of the population.
This is unlikely to satisfy either Poland or Spain. Both would lose some of the generous voting power granted to them by the Treaty of Nice in 2000. To compensate, both want the population threshold set higher to make blocking decisions easier. Both argue this is necessary to contain the power of the largest member states. Poland is also pushing for a special clause allowing four member states representing 12 or 15 percent of the EU population to block any decision.
Meanwhile, a group of 13 smaller and medium-sized member states argue the population threshold has been set too high, making decisions hard to take. They also say the high threshold increases the power of the larger countries.
Britain goes to the summit trying to ensure member states will not give up any veto powers in foreign, defense, tax, or social policy. These British "red lines" are likely to be respected.
At the same time, following the terrorist attacks in Madrid on 11 March, member states have agreed to a greater pooling of sovereignty in criminal justice matters. a new pan-EU prosecutor's office is expected to be set up.
A new European foreign minister's post is also likely to appear.
Whether all member states will retain a representative on the EU's executive -- the European Commission -- is another contentious issue. Under Irish compromise proposals, the size of the commission would be reduced to 18 in 2014. The seats would be distributed on the basis of equal rotation among member states.
A more immediate concern is the nomination of a new European Commission president. The current head of the commission, Romano Prodi, will step down in October. The new incumbent is likely to come from the center-right of the political spectrum.
The constitution, if approved, would only come into force after ratification in all 25 member states.
The summit in Brussels will assess the success of the EU's counterterrorism measures. It will agree a raft of measures to strengthen and develop the bloc's defense policy.
It will encourage Bulgaria and Romania to step up preparations for EU membership. Croatia will get the nod to start accession talks in early 2005. However, diplomats say this means Croatia will not be able to catch up with Romania and Bulgaria to join on 1 January 2007.
Turkey will be told the EU will start accession talks with the country if the commission finds in October that the country meets the Copenhagen political entry criteria.
The summit will definitively endorse the inclusion of Georgia, Armenia, and Azerbaijan in the European Neighborhood Policy. A draft statement, seen by RFE/RL, says that this will give the EU "another important instrument to promote progress on the wider reform agenda in each of those countries."
The draft summit conclusions also say that Belarus will be incorporated into the program once its had "established a democratic form of government, following free and fair elections."
EU leaders will call on Russia to sign and ratify border agreements with Estonia and Latvia.
The summit will also adopt statements on Iran, Iraq and the wider Middle East.