Mr al-Sadr, a long-standing critic of the presence of US troops in the country, will walk out from government in an attempt to press prime minister Nouri Maliki for a timetable for US troop withdrawal.

Yet the prime minister believes there is no need to implement a timetable at the moment and insists that a strict plan will not be able to cope with any increases in insurgency or violence in the wartorn country.

Nassar Rubaie, the leader of the al-Sadr bloc, said an "order" had been issued to all ministers within the bloc "to immediately withdraw from the Iraqi government".

"The six ministries shall be handed over to the government itself, hoping that this government would give these responsibilities to independent bodies who wish to serve the interests of the people and the country," he said at a news conference in Baghdad as part of a statement from Mr al-Sadr.

Six ministers in Mr Maliki's Shia-dominated government are now set to leave with Mr al-Sadr – a move that is unlikely to collapse the Iraqi government but will increase tensions in the national unity administration.

However it is thought that only the six ministers will resign. The movement has 32 members in Iraq's 275-seat government who are expected to remain in place.

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