By Jim Muir
BBC News, Baghdad
The decision by radical Shia cleric Moqtada Sadr to order his six ministers to quit the Iraqi government was not a surprising development.
Mr Sadr himself has not appeared - the US says he is abroad
Although electorally allied to Prime Minister Nouri Maliki's own al-Daawa party and partners in the big Shia coalition which dominates government and parliament, the two sides had been increasingly at odds as the pressures in the country mount.
Nobody expects Mr Sadr's move to bring the government down. Nor did observers believe that was his intention.
Rather than leave the cabinet seats empty, he himself suggested that the six abandoned portfolios be given to non-partisan independents, and some of his aides urged that competent technocrats be appointed.
That gesture was welcomed in a statement from Prime Minister Maliki, who also said he appreciated the Sadr movement's support for the political process.
The prime minister owes his position to the firebrand young cleric, who tilted the balance in his favour in the internal voting within the Shia alliance that produced Mr Maliki as its candidate for the job early last year.
Mr Sadr's decision to withdraw from the government appears to have been triggered primarily by its failure to heed the big popular demonstration he called in Najaf a week earlier to demand a withdrawal of coalition forces.
Hundreds of thousands attended the Najaf rally against the US
Hundreds of thousands of people, mainly Shias, attended the demonstration, waving banners and chanting slogans calling for the Americans to leave. It passed off peacefully.
The Sadr movement's demand for a withdrawal timetable has been turned down by Mr Maliki, who said last week during a visit to Japan that the timing would depend on when the Iraqi forces were ready to take full responsibility for security nationwide - a position he reiterated in his reaction statement to the Sadrist pullout.
The Sadr bloc has 32 of the 275 seats in the current parliament, and intends to continue its activities there and in the Shia coalition, despite withdrawing from government.
Another member of the Shia coalition, the Fadhila party, announced early last month that it was pulling out of that alliance because of the government's poor performance and sectarian quota composition.
But only if other major factions such as the main Sunni bloc and Iyad Allawi's secular Iraqi List were also to walk out of government, would it be at risk of collapse.
Nonetheless, the Sadr movement's defection from cabinet is likely to increase the pressure on the embattled prime minister.
Many ordinary Iraqis agree with his movement's assessment of the government's failure to provide services - another of the strictures cited in the pullout statements - as well as its demand for the withdrawal of foreign troops.
As the Najaf demonstration showed, the Sadrist movement enjoys strong grassroots support among the Shia masses, which make up 60% of the country's population.
The US is unlikely to mourn the loss of the Sadrists from the Iraqi cabinet
That support is not likely to be diminished by the group's withdrawal from government.
By distancing himself and his followers from the government over the issue of the foreign troop presence, Mr Sadr increases the pressure on Mr Maliki to show that his policy of cooperating with the Americans is paying off.
The US is unlikely to mourn the loss of the Sadrists from the Iraqi cabinet, despite its general desire for the government to be as broad-based as possible.
Washington regards Moqtada Sadr and his Mehdi Army militia as an even bigger threat to Iraqi stability than the Sunni-based insurgency.
Out of sight
The Mehdi Army has been blamed for many of the sectarian revenge killings of Sunnis that erupted after the bombing of the Shia shrine at Samarra in February last year.
Since the current security "surge" was launched by thousands of extra US and Iraqi troops in Baghdad over two months ago, Mr Sadr has dropped out of public sight.
Sadrists want at timetable for withdrawing US troops from Iraq
The Americans believe he has taken refuge in neighbouring Iran. Some Iraqi officials say he is in Syria. His followers insist he is in Iraq, but he did not appear at the Najaf rally which he had called.
The Mehdi Army militia, which once virtually ran the huge Sadr City suburb in east Baghdad, has melted off the streets since the surge began.
But some of its fighters were engaged in several days of clashes last week with US and Iraqi troops in the town of Diwaniya, south of Baghdad.
They have also clashed several times recently with British forces at Basra in the south of the country. The degree of Mr Sadr's control over some elements of the Mehdi Army has been questioned.
Whatever the temporary fate of his militia fighters in one area or another, Moqtada Sadr's influence makes him one of the most significant players on the Iraqi stage, partly because of the mantle he has inherited from a revered clerical family.
That influence is unlikely to be historically diminished by his decision to pull out of the current Iraqi cabinet.