The followers of Muqtada al-Sadr, who form a king-making bloc in the next Iraqi government, have confirmed they will not accept Prime Minister Nour al-Maliki's candidacy for a second term as leader.

The move effectively ends the career of the US-backed incumbent. The Sadrists' stance comes after a five-month political stalemate that threatens to leave Iraq leaderless, as the remaining 15,000 or so US forces leave the country ahead of a 31 August deadline.

The move is seen as a potential breakthrough in the destabilising deadlock that many believe has led to a steady increase in violence. The decision was confirmed today by the head of the Sadrist political bloc, Nassar al-Rubaie. He said he had told Maliki that he would have to leave his post in order for the Sadrists to maintain support for his party in an eventual coalition government.

Without the support of the Sadrists, who are led by the exiled anti-western cleric, Muqtada al-Sadr, neither of the two biggest rival political blocs would be unlikely to be able to form a government. This includes the head of the Iraqiya Alliance, Iyad Allawi, who last month courted al-Sadr's support in a public meeting in the Syrian capital, Damascus.

However, al-Rubaie indicated that the Damascus summit has not led the Sadrists to split from the largely Shia Islamic bloc that threatens to sidelineAllawi's cross-sectarian alliance.

Al-Rubaie revealed that the 40-strong bloc of MPs in an eventual new parliament had thrown their support behind former transitional prime minister Ibrahim al-Jaafari, who led Iraq throughout 2005, on the eve of the breakout of full-blown sectarian chaos.

"Maliki eventually isolated all around him by his behaviour," said al-Rubaie. "This includes all members of his alliance. In the end, it was only his Dawa party that had influence that spread throughout all branches of government.

The Sadrists have had a strained recent history with Maliki whom they accuse of allowing US jets to bomb Sadr City in Baghdad in 2007 and 2008 and of reneging on a pledge to release up to 400 Sadrist prisoners who were handed over by US forces late last year.

"Whenever we asked him anything, he would say yes, yes, yes, then not keep any one of his pledges," said al-Rubaie.

The move against Maliki was announced yesterday by former deputy prime minister, Ahmed Chalabi, whose Iraqi National Congress Party has allied with the Sadrists and the main party in their bloc.

Both Chalabi and al-Jafari remain deeply unpopular with the US government. They accuse Chalabi of providing false information that helped make a case for the 2003 invasion. The US gripe with al-Jafari, however, is that he failed to safeguard the country from a slide into sectarian crisis.

Iraqi officials said yesterday that July marked the highest nationwide death toll of any month since the same period in 2008. They said 535 people were killed and 1,043 wounded during violent attacks.

However, the US military later refuted that figure, claiming only 222 people were killed last month, less than half the number compiled by Iraq. US officials have dreaded a scenario of Iraq remaining without a new government after their combat forces have left by the end of the month. The US have been equally sensitive to claims that violence is increasing and that services have barely improved throughout over the past seven years.