Ayatollah Sistani, one of Iraq's most senior Shia clerics, will not endorse any political groups for December's election, his spokesman has said.
Ayatollah Sistani is a leading player in post-Saddam Iraq
The grand ayatollah wants Iraqis to vote according to their beliefs, Sheikh Abdul Mahdi al-Karbalai said in a sermon on Friday.
The ayatollah's statement may worry the ruling Shia-led coalition, the United Iraqi Alliance.
His support before the January election helped them win over many Iraqi Shia.
Delivering a Friday sermon in the holy city of Karbala, Sheikh Karbalai revealed that Grand Ayatollah Ali Sistani, a marja, or source of emulation for his followers, would not back any party.
"The marja enjoins Iraqis to participate massively in the forthcoming elections, but does not support any political group in particular," he said.
"It's up to Iraqis to make their choice based on their beliefs."
He also questioned the motives of those who did not vote in the constitutional referendum on 15 October. The turnout among Iraqi Shias was lower than expected.
"We say to those who did not vote in the referendum on the constitution: What have they gained? How can they consider themselves Shias and not obey their religious leaders?"
Ayatollah Sistani's neutrality may have a significant impact on the ruling Shia-led United Iraqi Alliance.
The BBC's Middle East analyst, Roger Hardy, says it could make December's election a more open race.
On Thursday it joined Iraq's other political parties in finalising its coalition for December's planned general election, ahead of Friday's deadline for registration.
Ayatollah Sistani endorsed the UIA in January's election
The ruling Shia Islamist parties only agreed to register as a united bloc after a last-minute agreement on Thursday evening.
The two main Kurdish parties - the Democratic Kurdistan Party and the Patriotic Union of Kurdistan - will also be running together, they announced.
While the main Shia and Kurd alliances that contested the previous election in January remain much the same, other groups will also fight the election.
Three Sunni parties that boycotted the vote in January have set up a coalition, the Iraqi Accord Front.
The new Sunni alliance, announced on Tuesday, called on Iraqis to take part in the December's poll and to reject any calls for a boycott.
A fourth challenge for votes will come from the Shia former Prime Minister Iyad Allawi, who has expanded his secular list to include Sunni personalities, as well as communists and liberals.