The alteration was made by the country's National Assembly, which is dominated by Kurds and Shias. Sunnis, who are campaign-ing against the federal constitution, accused the assembly of manipulating the rules to ensure a "yes" vote. They are now threatening to boycott the vote, due to take place on 15 October.
Under the old rules the constitution could be rejected if two-thirds of those polled voted "no" in three provinces. Sunni leaders had maintained they would be able to achieve this. The change means a "no" vote would require two-thirds of all registered voters to reject the constitution.
A UN legal adviser, Jose Aranaz, said: If this goes ahead it would seriously compromise the exercise."
The Sunnis maintain that a planned federal structure will leave the oil-rich north and south in, respectively, Kurdish and Shia hands and lead to the dismemberment of the state.
The development came amid continuing violence around the country and the arrest of a Briton among 10 suspected terrorists. Iraqi police named the British suspect as Colin Peter who was held between Anbar and Najaf allegedly carrying machineguns and satellite navigation equipment in a convoy of vehicles.
He claimed to be a contractor but police said his passport did not support his claim. The arrest was confirmed by the British military in Basra.
Five American soldiers were killed yesterday as US and Iraqi forces continued a major offensive against insurgents in western Iraq. In Baghdad, a suicide car bomber penetrated the Green Zone and killed two soldiers and a civilian.Reuse content