The instructions from al-Sistani, a reclusive spiritual figure with a strong influence among the Shia, fall short of a religious edict, but still carry tremendous weight.
It could dent the hopes of Iyad Allawi, a former prime minister whose secular Shia non-sectarian party is mounting a challenge to the ruling coalition.
A representative in al-Sistani's office on Saturday said he had instructed followers to do three things: turn out to vote on the day; avoid voting for any list whose leader is not religious; and avoid voting for "weak" lists so as not to split the Shia vote.
The instructions looked like a coded endorsement of the United Iraqi Alliance, the main Shia list which won the last election in January and dominates the current government.
United Iraqi Alliance
The United Iraqi Alliance groups Iraq's two most powerful Shia parties - the pro-Iranian Supreme Council for the Islamic Revolution in Iraq (SCIRI), and the Dawa party, which is headed by Ibrahim al-Jaafari, the prime minister.
The alliance was formed for January's election with the blessing of al-Sistani. His aides have since criticised the government's performance and he has distanced himself from party politics, telling his aides not to give explicit endorsements.
Prime Minister Ibrahim al-Jaafari
leads the Dawa party
But as well as now appearing implicitly to back the alliance, al-Sistani's instructions may turn Shia voters away from a rival list headed by Allawi, who is secular and has built a coalition of Sunnis, Shias and Kurds.
Al-Sistani's statement may also damage a list headed by Ahmad Chalabi, the deputy prime minister, another secular Shia.
It encourages voting for the most powerful bloc, rather than smaller parties or lists, which is likely to favour the United Iraqi Alliance over smaller Shia Islamist groups.
Since January's election, some small Shia groups, also religiously based, have broken away from the United Iraqi Alliance and registered independently for the polls. Al-Sistani, who is in his late 70s, rarely speaks publicly, but statements issued by his office in Najaf are considered authoritative.