Large numbers of Sunni Arab voters turned out in Mosul
Two Sunni-dominated provinces in Iraq have rejected the country's draft constitution, according to partial results given by election officials.
Electoral rules mean the document will fail if three out of the 18 provinces vote "No" by two-thirds or more.
Salahuddin and Anbar both heavily voted against but Diyala, also Sunni, has backed the charter.
Now all eyes are on the largely Sunni province of Nineveh where the result is due to be announced within two days.
The BBC's Jim Muir reports from Baghdad that the referendum has turned into something of a cliff-hanger.
If the constitution were to fail, it would set the political process back by roughly a year.
New elections will be held in December and the resulting parliament would again undertake the task of producing another constitution, our correspondent says.
In Anbar, 97% of voters cast "No" ballots while it was 82% against in Salahuddin, electoral commission chief Abdel Hussein al-Hindawi told reporters, quoting preliminary figures.
CONSTITUTION'S KEY POINTS
Iraq to be federal, parliamentary democracy
Official languages to be Arabic and Kurdish
Official religion to be Islam but religious freedoms guaranteed
Equal rights for all
Elections every four years
In Diyala, the constitution was approved by more than 50% of voters.
Mr Hindawi added that the commission had so far received 135 minor complaints over violations which did not affect the results.
Nineveh has a religiously and ethnically mixed population, dominated by Sunni Muslims.
In the immediate aftermath of the referendum, election officials in the provincial capital, Mosul, were quoted by an international news agency as saying the "Yes" vote had won by a huge majority.
Most impartial observers were perplexed and perturbed, the BBC's Richard Galpin reports, as the word on the street seemed to be that the majority had in fact voted "No".
But it was not clear, our correspondent adds, if the "No" voters had mustered two-thirds.
Nineveh is one of the provinces under investigation by election officials. They are looking at voting procedures, the ballot boxes and the ballot papers to ensure there were no mistakes or fraud.