The law, backed by some Shia leaders who have been keen to set up a big, autonomous region in their oil-rich south, was passed in a session of parliament on Tuesday.
The vote was boycotted by the Accordance Front, the largest political bloc of the Sunni Arab minority.
Hostility between rival political parties over federalism - one of post-war
Legislators loyal to radical Shia leader Moqtada al-Sadr and the smaller Shia Fadila Party stayed away from Wednesday's vote, showing Shia support for federalism is not unanimous.
Shia and Sunni leaders last month agreed to put off implementation of the federalism law for at least 18 months, effectively delaying the creation of any autonomous regions until 2008.
The biggest Shia party, SCIRI, proposes merging nine of
Sunnis want amendments to the constitution that keep the wealth distribution power in the hands of the central government.
Photo Caption
The Iraqi Parliament