Six ministers from Moqtada al-Sadr's political group have withdrawn from Iraq's coalition government.
Mr Sadr's bloc is the largest single political group in Nuri al-Maliki's coalition, but the prime minister will be able to hold on to power if he keeps the support of smaller Shia and Kurdish groups.
Nassar al-Rubaie, flanked by allies from his 32-strong parliamentary bloc, announced the withdrawal at a Baghdad news conference, reading a statement from Moqtada al-Sadr.
He said the main reasons for the withdrawal were the prime minister's lack of response to the demands of nearly one million people in Najaf asking for the withdrawal of US forces and the deterioration in security and services.
On 9 April, a rally organised by the group saw huge crowds gathered in Najaf to demand the withdrawal of foreign troops, though the number of protesters was disputed, with the US military reporting around 15,000 and Iraqi officials saying it was in the hundreds of thousands.
Mr Maliki welcomed the Sadr boycott, saying it would help speed up his long-awaited cabinet reshuffle that would bring in 'efficient ministers.'
His adviser, Mariam al-Rayis, said that by boycotting the government the Sadr bloc was emerging as a useful opposition voice in the parliament.
Further insurgent attacks today
Insurgent activity continued during the day, with at least 18 people killed in northern Iraq in separate attacks, including 13 Iraqi soldiers who were shot dead by gunmen who attacked their checkpoint.
The attack took place in a village southwest of Mosul.
Two university academics were also shot dead by unidentified gunmen in Mosul and three others were killed in a car bomb blast north of Baghdad.
In another incident, gunmen kidnapped ten employees of the education ministry as they were travelling to Baghdad from the northern oil city of Kirkuk.
Earlier, US forces killed three Iraqi policemen in a case of 'friendly fire' during a raid against what they described as suspected al-Qaeda militants in Iraq.
The US military said in a statement that its ground forces had returned fire having been fired upon during an operation in Ramadi, west of Baghdad.
Three men killed in the confrontation were later identified as Iraqi police.
The military said its troops had coordinated their operation and no Iraqi police were known to be in the area.
The incident is under investigation.
