While Moqtada al-Sadr, Iraq's 30-year-old Shia firebrand, leads his insurrection from the holy city of Najaf, Ali al-Sistani - the spiritual leader of Iraq's 15m Shia Muslims - sits silently in London, recovering from minor heart surgery. We are watching two battles for Najaf: the physical fight raging between Mr Sadr's Mahdi Army and the US Marines, and a subtler struggle for influence within Iraq's Shia majority - a struggle that is in many ways a battle for the future of the country.

Najaf, as the resting place of Imam Ali, the founder of the Shia faith, is the holiest city on earth for the world's 200m Shia Muslims. The leaders of the city's clerical community are the natural leaders of the global Shia community, whose members everywhere aspire to be buried in the cemetery where so much of the fighting has been taking place. Najaf's cemetery contains millions of graves and dominates the city like the white beard on an ayatollah's face.

Mr Sistani is the most senior of Najaf's four grand ayatollahs and doubts about his health have highlighted the succession issue. Regardless of timing, any successor to Mr Sistani as the religious leader of the Shia in Iraq could come only from among the other three. All three are, like Mr Sistani, from the "quietist" school of Shia leadership which eschews the sort of direct political rule exercised by the mullahs of Iran. All are in their 70s.

The only native Iraqi among them is Mohammad Said al-Hakim. Some observers believe he would lean toward the policies of the Supreme Council for the Islamic Revolution in Iraq, a Shia political group run by a cousin of Mr al-Hakim's that has historic ties to Iran. The council allied itself with the US-led coalition during the war and has been relatively cooperative with the occupation since then. Bashir Najafi, a Pakistani who has been in Iraq most of his life, has been an outspoken critic of the occupation and, as the most radical and anti-American of Mr Sistani's potential successors, could be expected to take a more activist line than Mr Sistani. The third grand ayatollah is Ishaq Fayyad, an Afghan who has been in Najaf since the age of 10. Mr Fayyad is known as a "reformer's reformer" and, of the three, is the most committed opponent of Iranstyle clerical rule.

Mr Sistani's successor would be chosen according to an informal and ever-fluid process involving the various levels of the Shia community. Every lay Shia - every taxi driver in Beirut or Hazara shepherd in Afghanistan - chooses one top cleric as his source of guidance on matters of spiritual jurisprudence. The outgoing leader often anoints his successor with whispers in the ears of colleagues. And the senior clerics, including maybe two dozen ayatollahs, have the greatest sway of all. No part of this process is formal or transparent, no front-runner has emerged and nobody knows for how many weeks or decades Mr Sistani will survive the three blocked arteries that caused his trip to London. What is certain, however, is that the conditions feeding the rage of the urban poor who currently dominate Iraqi Shia politics will not disappear in the near future.

Iraq's slums - places such as Sadr City in Baghdad - are some of the most demoralising places on earth, sun-baked urban wildernesses of cinderblock and wire where families live seven or nine to a room and sewage and engine oil bake in the gutters. There are no jobs in these places and seemingly no prospects. The feeling of impotence is pervasive.

After 30 years of Sunni apartheid under Saddam Hussein, Shia are impatient for change. They were betrayed by the British in 1930, with the arrival of a foreign prince who ruled through the Sunni minority. They were betrayed by the US in 1991, when they followed George H.W. Bush's exhortations and rose against Mr Hussein, only to be crushed. They form 60 per cent of Iraq's population and have been waiting centuries to run their own affairs. They cannot understand why Mr Hussein's fall seems to have brought so little improvement to their lot.

Mr Sadr, who draws his support almost exclusively from the disaffected male youth of Iraq's Shia slums, enjoys exclusively sectarian support and has no religious authority. He is between 24 and 30 years old, depending on who is counting, and it is debatable whether he has even completed his seminarian training. Even if he had the requisite juridical talent, it would take him 30 or 40 years to become a grand ayatollah.

Mr Sadr's temporal power, however, is substantial. His constituency consists of the majority of Iraqi Shia. A poll by the coalition authorities in May gave Mr Sadr 68 per cent approval nationwide. However, only 2 per cent backed Mr Sadr for Iraq's presidency. In other words, he attracts much popular sympathy but is not taken seriously as a leadership choice. His standing comes partly from the enormous prestige of his father, a grand ayatollah killed by Mr Hussein's people in 1999. Most important, Mr Sadr represents a set of grievances and aspirations embodied in a movement that exists with or without him, whatever his fate as the Marines fight their way ever closer to the Imam Ali shrine where he has taken refuge in Najaf.

Mr Sistani's silence while battle blazes through Najaf - his home and the wellspring of his authority - is not as remarkable as it might seem. For all the "quietism" that has seen him shun formal political roles, Mr Sistani is a very active political player. He has imposed prior truces in Najaf and Kerbala, scuppered US plans for regional caucuses in the constitutional process, forced the June 30 date for the handover of sovereignty and dictated the abandonment of federalism in the latest United Nations resolution. "Quietism" is different from "quiet", and Mr Sistani's current silence isaloud, clearrefusal to rescue a man whom senior Shia see as an unruly thug, and who is a rival to Mr Sistani for sectarian authority in the Shia community.

Whoever emerges as the main channel for Shia political energy in Iraq, the thrust of that energy will be the same: maximum power for the majority via a maximum of direct democracy; as little federalism as possible; and as much Islam as possible. For Iraq's minorities - Sunni Arabs, Turcomans, Christians and Kurds such as Nechirvan Barzani, prime minister in Iraq's Kurdish north, who once told me that Iraq was a "voluntary union" - the Shia project is alarming. When the US-led coalition finally goes, the question of whether the Shia can deliver the conditions for a lasting, unified country will be determined by the winner of the real battle for Najaf.