By Jim Muir
BBC News, Baghdad
The Iraqi parliament has approved a law allowing provinces to merge into regions which would enjoy a measure of autonomy.
Abdulaziz Hakim said the Iraqi people would have the last word
The law is controversial as many Sunni Muslims and others fear it would lead to the country's partition.
The vote went through unanimously, but only 138 of the chamber's 275 members were present.
Absentees included the two biggest Sunni blocs and two of the factions that make up the big Shia alliance.
There were some confused scenes in parliament as the controversial bill was read through clause by clause.
There were many significant absentees. Two of the factions which make up the big Shia alliance - Moqtada Sadr's group and a smaller one called Al Fadhila - also boycotted the proceedings.
Spokesmen for these groups later said they were totally opposed to the federal region's law.
The Sunni group said they feared it heralded Iraq's fragmentation.
Some Shia spokesmen said they believed it would have a negative impact on the political process and on hopes for national reconciliation.
But Abdulaziz Hakim, the leader of the biggest Shia faction, the Supreme Council for the Islamic Revolution in Iraq, described it as a blessed day.
His and some of the other Shia groups have been pressing for the law.
They hope that the Shia-dominated south can set up a federal region something like that already being run by the Kurds in the north who also strongly back the new law.
But Mr Hakim said that the Iraqi people would have the last word.
Any provinces wanting to join together into a federal region will have to seek popular approval though a referendum.