They said the proposals, which include some tax issues, would be unveiled on Wednesday and sent to EU capitals before a summit in Helsinki next month. Enlargement is the EU's most ambitious project but risks becoming its biggest problem as it threatens to dilute the process of European integration. The proposals could widen divisions between those EU partners seeking closer integration and those fearing a creeping federalism.
The EU has to streamline its internal procedures if it is to have a workable structure with up to 27 member states in the years to come. It has set up an intergovernmental conference (IGC) next year to adapt its institutions for enlargement.
The full IGC agenda will be formally launched in January, but the Commission's proposals largely echo a blueprint produced last month by a group of three "wise men".
They include extending qualified majority voting to areas of cooperation between police and judicial authorities, such as visa and immigration rules, as well as to certain areas of taxation, like the environment.
This is likely to prove difficult for some EU partners, led by Britain and Luxembourg, who want to maintain unanimity voting on tax issues.
Commission sources said Chris Patten, one of the two British Commissioners, opposes the plan but would not block its adoption by the 20-seat executive. Under the qualified majority voting system, the vote of each EU member state has a weighting reflecting its population.
The Commission will also propose a reweighting of each member state's vote. The current system favours the smaller countries. The plans will not detail at this stage how any new weightings will be calculated to reflect a country's population, the sources said.
Nor will there will be precise plans on the size of a future executive where the bigger states currently have two seats each, with one apiece for the 10 smaller partners.
The Commission will support moves to give greater powers to the president, currently Italy's Romano Prodi, and reinforce his powers to demand the resignation of any individual commissioner.
The proposals will also make it easier for those member states wanting to forge ahead in some areas without being held back by the waverers.