As Shia and Kurdish factions presented the document to the National Assembly, minutes before a midnight deadline, Sunni Muslims strongly opposed to its federal structure made accusations of "betrayal" and warned of a violent sectarian backlash. A vote on the draft was later delayed for three days in the hope that the sides could come to an agreement on its wording.
The draft constitution is the principal plank of President George Bush's exit strategy from the Iraq conflict, which has made his popularity collapse among American voters.
American diplomats, led by the ambassador, Zalmay Khalilzad, had been frantically lobbying for it to be adopted before last night's deadline. But far from sealing Iraq's post-Saddam era, the draft appeared to be quickly fracturing the fragile edifice of government, with Shia and Kurdish parties declaring they were prepared to use their parliamentary majority to push through the document in the teeth of Sunni opposition.
The Sunni reaction was immediate, with politicians queuing to denounce the move and warning of a cataclysmic reaction. Soha Allawi, one of the leading negotiators, declared: "We will not be silent. We will campaign for public awareness to tell both Sunnis and Shias to reject the constitution, which has elements that will lead to the break-up of Iraq and civil war." Another Sunni delegate, Hussein Shukur al-Fallu, said: "If they pass this constitution, then the rebellion will reach its peak."
Sunni leaders said the text had dropped wording that forbade secession from Iraq; Kurdish parties maintained they did not want to break away entirely but wanted to keep the option open.
There were also warnings from Sunni insurgent groups, engaged in a war of attrition, that they will increase their attacks, targeting those responsible for the constitution.
But some militant Shias, including followers of the radical cleric Muqtada Sadr with their powerbase in relatively resource-poor central Iraq, are also opposed to federalism and yesterday renewed their call for "Iraqi unity". In a further sign of growing polarisation, several minority and tribal groups also said guarantees made about their roles had been changed in the draft document.
A spokesman for the tribal umbrella group said: "The text of the constitution was destroyed in violation of what it had been agreed on. We shall now boycott the political process." Mohaim Ased Abdul, the chairman of the Assembly of Minorities, added: "We must oppose this because it does not represent minorities."
Yonadem Kanna, a representative of Iraq's dwindling Christian community, said he expected Sunni leaders to start mobilising their supporters against the constitution. "Tomorrow on the street, on the ground, they will move against the constitution, that we can say for sure."
There was also controversy over the role of Islam in a future administration, with the main Shia party insisting it should be the main source of law and womens' groups warning that it would lead to the denial of female rights.
The most contentious issue in the document was federalism, which the majority Shia and Kurdish factions are determined to make the basis of government.
The Sunnis, who have already seen their dominance under Saddam and previous regimes overturned in elections this year, are convinced this is a pretext for the Shias and Kurds carving out the oil-rich regions in the north and south of the country.
A copy of the document, seen by the media, described the future Iraq as a "republican, parliamentarian, democratic and federal state" without specifying the exact nature of the federalism. The draft needs to be approved by a majority of the 275-member National Assembly, but Hussain al-Shahristani, the Shia deputy speaker, insisted it would be passed with a substantial majority. If approved, the constitution will be put to a referendum on 15 October; it can become defunct if any of the 18 provinces reject it by two-thirds or more.
Jalaaldin al-Saghir, a Shia negotiator, said: "There is a time limit and we do not want to breach it. We had talks with our Sunni brothers. We cannot wait for all the time needed by those people to be convinced. We agree that the constitution, including federalism, be put before the people. If the Sunni Arabs do not want to vote for federalism, they can reject the constitution."
Mr al-Saghir said Shias and Kurds had also agreed that no laws would be allowed to contradict the principles of Islam. He said: "In addition, no law shall be adopted that contradicts human rights and democratic principles. Also it was stated that the constitution ensures the Islamic identity of the majority of Iraqi people."
Meanwhile, violence has continued unabated. Yesterday, gunmen killed 10 people, including eight policemen, in a van north of Baghdad, and two American soldiers were killed in a bomb attack near Samarra.
As talks continued into last night, the talk of insurrection and of a steadily deteriorating situation continued to grow.