Jaafari won 24 percent of the 1.43 million votes cast on Friday and Saturday in a plebiscite to pick a prime minister.

The vote was an attempt to clarify Iraq's muddled politics after a March 7 parliamentary election that failed to produce a clear winner.

Iyad Allawi, another former prime minister whose cross-sectarian Iraqiya coalition won the most seats, finished fifth in the vote, Sadrist officials said. Current Prime Minister Nuri Al-Maliki, whose largely Shiite State of Law bloc finished second in the election, placed fourth.

The referendum had no government sanction and is not legally binding. However, party officials said they would adhere to the results as they negotiate a nominee for prime minister.

The Sadrists' strong showing in the parliamentary vote — the movement won around 40 seats as part of the Shiite Iraqi National Alliance (INA) bloc — made Sadr a potential kingmaker in what are expected to be weeks of talks to form a government.

Jaafari, a Shiite, served as prime minister in the transitional government in 2005-06 following the US invasion that ousted Saddam Hussein.

With no clear election winner, coalitions are jockeying for alliances that could give them the parliamentary majority needed to form a government.

INA, whose leaders have close ties to Iran, won a total of 70 seats and has talked with State of Law about a possible merger of the two largest predominantly Shiite blocs.

But the current prime minister is known to be a stumbling bloc. The Sadrist movement objects to a second term for Al-Maliki, who sent troops to crush Sadr's Mehdi Army militia in 2008.

The referendum ballot carried five names plus a space for a write-in candidate.

Jaafar Mohammed Sadr, a relative of the cleric whose name was on the ballot, finished second with 23 percent. Qusay Abdul-Wahab Suhail, a write-in candidate, placed third with 17 percent.

Party officials said the referendum was open to all Iraqis but it was not clear how widely the ballots were available outside Sadrist strongholds in Baghdad and elsewhere.