Under the treaty, it is envisaged that a rotation system of commissioners will operate, writes Denis Staunton, European Correspondent
The European Commission is the central institution of the EU. It develops proposals for EU laws and policies and acts as guardian of the treaties.
Although any new laws must be approved by member-states meeting in the Council of Ministers (and sometimes by the European Parliament), it is the Commission's job to implement and enforce them.
Since the creation of a single market, the Commission's role as the EU's competition authority has become increasingly important.
Although its members are appointed by the member-states, the Commission acts on behalf of the EU as a whole.
At present, each of the biggest five member-states - Germany, France, Britain, Italy and Spain - appoints two members of the Commission and the others appoint one Commissioner each. The President is chosen unanimously by the heads of government of the 15 member-states.
In Amsterdam five years ago, the big countries agreed to give up their second Commissioner if a wider package of reforms was agreed to prepare the EU to accept new members. This will take effect when the next Commission takes office in January, 2005.
Under the Nice Treaty, each member-state would nominate one Commissioner until the EU grows to include 27 countries. From then on, a rotation system will operate so that member-states take turns to lose their Commissioner for a five-year term.
"The number of members of the Commission shall be less than the number of member-states. The members of the Commission shall be chosen according to a rotation system based on the principle of equality, the implementing arrangements for which shall be adopted by the Council, acting unanimously. The number of members of the Commission shall be set by the Council, acting unanimously," the treaty says.
The precise nature of the rotation system has yet to be agreed but it will operate on a strict equality basis so even the biggest countries will have to take their turn.
"Member-states shall be treated on a strictly equal footing as regards determination of the sequence of, and the time spent by, their nationals as members of the Commission; consequently, the difference between the total number of terms of office held by nationals of any given pair of member-states may never be more than one," the treaty says.
It also specifies that each successive Commission "shall be so composed as to reflect satisfactorily the demographic and geographical range of all the member-states of the Union."
Critics argue that removing the automatic right of each member-state to appoint a Commissioner will undermine the Commission's legitimacy, especially in smaller states. But the treaty's supporters say that a Commission with more than 27 members would prove unwieldy and ineffective.
If the Nice Treaty comes into force, the Commissioners and the Commission President will no longer be chosen unanimously but by qualified majority voting. This will mean a country cannot veto a nominee who is the choice of almost everyone else.
But it also means that each member-state's nominee for the Commission will not be accepted automatically. The President will become more powerful, able to move Commissioners to different portfolios and, if the other Commissioners approve, to sack them.