The Iraqi Islamic Party (IIP) had earlier agreed to endorse the draft constitution once a key demand - that parliament reviews possible amendments to the constitution four months after the 15 December parliamentary election - was met.
There was no agreement, however, about precisely how the constitution might be amended, leaving the Sunni Arab community still at risk of being disappointed in next year's negotiations.
The Shia deputy speaker of parliament, Hussain Shahristani, said he expected Sunni Arabs, who mostly boycotted a January election that then sapped their power to influence negotiations on the constitution in parliament, to take part in large numbers in the expected 15 December elections.
But the Association of Muslim Scholars (AMS) believes the last-minute amendments fell far short of their conditions for endorsing the draft paper.
AMS Secretary-General Shaikh Harith al-Dhari, after meeting an Arab League delegation on 11 October, listed the group's conditions as a timetable for the pullout of foreign forces, a definition of terrorism and recognition of the Iraqi resistance as well as an effort to reinstate the Iraqi army.
The AMS on Thursday regretted the IIP's support for a constitution which it said would "fragment Iraq and destroy its identity".
Iraqi heads are to vote on the
draft constitution on Saturday
AMS public relations official Abd al-Salam al-Kubaisi said it was not possible to accept a constitution supervised by the US forces and written with martyrs' blood.
Earlier, IIP officials had defended their support for the new charter after the introduction of new amendments.
"During the conference that we held with the AMS, a resolution, endorsed by the meeting, had said if the constitution was submitted for referendum in its present form, it should be rejected. We were determined to say 'No' to the draft if it remained in its present form," member of IIP political bureau, Naseer al-Ani, told Aljazeera.
''But, thanks to the pressure we exerted on those who drafted the constitution, we have got the opportunity to support the draft [by adding the amendment article]''.
Referring to the AMS fears that the party has been deceived and that there are no guarantees the amendment will materialise within the stated time, he said:
"What is important is that we have a whole article officially added to the constitution which makes the amendment an imperative. We are dealing with parties in a political arena based on negotiations and agreements. We want to reduce the tensions and minimise the presence of foreign occupation and the security apparatus that torment our people."
"What is important is that we have a whole article officially added to the constitution which makes the amendment an imperative"
IIP political bureau member
On Sunni Arab divisions over current issues, al-Ani said: "These divisions are far away from our people as we all stand under the same ceiling and in the end will remain united."