While Sadr was instrumental in Maliki becoming prime minister last year, the move is unlikely to significantly weaken the government since Sadr's movement does not hold any key cabinet portfolios. It could actually help Maliki by giving him a freer hand to pursue his political policies.

At the same there will be concerns about keeping the anti-American cleric and militia leader engaged in the political process, even though the Sadrists said they would remain in parliament.

Washington has called the Mehdi Army, a Shi'ite militia that claims loyalty to Sadr, the biggest threat to Iraq's security.

The Sadrists accused Maliki of "ignoring the will of the people" over the timetable issue and also failing to improve basic services and effectively deal with deteriorating security. Baghdad's Sadr City slum is the cleric's main powerbase.

"The prime minister has to express the will of the Iraqi people. They went out in a demonstration in their millions asking for a timetable for withdrawal. We noticed the prime minister's response did not express the will of the people, " the head of the Sadrist bloc in parliament, Nassar al-Rubaie told a news conference.

Tens of thousands of Iraqis answered a call by Sadr to rally in the holy Shi'ite city of Najaf last week to protest against the presence of more than 140,000 U.S.-led forces in Iraq. Sadr himself did not appear - U.S. officials say he is in hiding in Iran, while his aides say he is still in Iraq.

Maliki said afterwards he saw no need to set a timetable. He said his government was working to build up Iraq's security forces as quickly as possible so U.S.-led forces could leave.

"For the public benefit and lifting the suffering of the patient Iraqi people ... we found it necessary to issue an order to the ministers of the Sadrist bloc to withdraw immediately from the Iraqi government," Rubaie said, reading a statement on behalf of Sadr.

Internal Dissent

One analyst said Sadr could be acting to quell internal dissent over his support for a U.S.-Iraqi security crackdown which has failed to stop car bombings blamed on Sunni Islamist al Qaeda that have targeted Shi'ite neighborhoods in Baghdad.

"Sadr is coming under pressure because of his tacit support of the security plan ... So he has to restore internal discipline, which he does by withdrawing from the political process and going back to the street," said Joost Hiltermann of the International Crisis Group think tank.

Before he entered mainstream politics, Sadr's Mehdi Army fought two uprisings against the Americans in 2004. Since then, the militia has been involved in tit-for-tat attacks against minority Sunni Arabs amid spiraling sectarian violence.

The militia has kept a low profile since the launch of the Baghdad security plan, reportedly on the orders of Sadr.

The Sadrists ended a two-month boycott of parliament in January after pulling out in protest over the timetable issue and a meeting between Maliki and U.S. President George W. Bush. They returned after a deal was brokered.

"Nobody really missed them - their seats were patronage places. Maliki will appoint new ministers and try to bring in people who are more competent," Hiltermann said.

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Additional reporting by Yara Bayoumy and Aseel Kami.

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