"Anyone who contacts the grand ayatollah's office is told that he instructs Iraqis to vote 'yes'" in Saturday's crucial vote, said an official from his office in the Shia holy city of Najaf.

Al-Sistani, who rarely speaks in public, was one of the driving forces behind the landmark January elections that saw the majority Shia community and secular Kurds head Iraq's new parliament.

Political leaders announced on Wednesday some modifications to the constitution and an agreement to consider further revisions after new elections in December to try to win over the Sunni Arabs, who make up about 20% of the population.

"The Iraqi faithful must go to the polling stations to vote in favour of the test after the changes approved on Wednesday which eliminate all the arguments" against the constitution, said Sayed Ahmed Safi, an al-Sistani representative in the holy city of Karbala.