Such stature seems all the more remarkable given that the Ayatollah himself is not Iraqi but a native of Mashad in northeast Iran.

A prodigy from a religious family who began learning the Koran at the age of five, he has spent almost his entire life in the intellectually rigorous atmosphere of Shia scholastic institutions, first in Iran and then, from his early twenties, in Najaf, the centre of Shia learning for the past thousand years.

Promising scholars would be expected to master philosophy and jurisprudence, mostly through debate, and Ayatollah al-Sistani also pursued a keen interest in modern science. Years studying grammar and rhetoric are reflected in his elegantly pure classical Arabic, although he has never lost his thick Iranian accent.

Leaders in the Shia hierarchy emerge, in part, on their ability to gain a following by virtue of their pronouncements on questions of religious law. Ayatollah al-Sistani also enjoyed the powerful support of the widely revered Grand Ayatollah al-Khoei, his teacher and predecessor as supreme religious authority. He shared his mentor's distaste for the political philosophy of Ayatollah Khomeini, who spent years of exile in Najaf before returning to Iran.

Grand Ayatollah al-Khoei died in 1992, and Ayatollah al-Sistani assumed responsibility for a flock devastated by Saddam's bloody reprisals for the Shia uprising after the 1991 Gulf War. Taking a low profile, he eschewed politics but still attracted a large following, thanks to the popularity of his rulings on law and personal behaviour.

Combining high and low technology, his followers around the world would e-mail requests for rulings to an office in the Iranian city of Qom. Such queries were then printed out and smuggled across the border to the Ayatollah's house in Najaf, and his answers smuggled back to Qom for posting on the Sistani.org website.

His moral authority among the poverty-stricken Shia masses was bolstered further by his generous distribution of financial contributions, while his own lifestyle remained rigorously austere. "You get just one glass of tea, and the mattresses you sit on are very thin," said one visitor.

Ayatollah al-Sistani remained politically aloof during last year's war, declining either to condemn or endorse the coalition's presence in Iraq. But last June he dropped a bombshell, issuing a ruling that declared the American plan to have a new constitution written by an unelected committee unacceptable and demanding that any new constitution be written by an elected assembly.

Eventually persuaded that this edict might be serious, Paul Bremer, then Iraq's American administrator, requested a meeting with Ayatollah al-Sistani, which was refused.

Mr Bremer then requested that the Ayatollah nominate representatives to meet his officials to negotiate a compromise. "Mr Bremer, you are American. I am Iranian. I suggest we leave it to the Iraqis to devise their constitution," the Ayatollah replied.

After the fall of Saddam, Ayatollah al-Sistani denounced looting, which rapidly died down in Shia towns and cities.

His representatives helped to organise local councils to enforce law and order and restore basic services. He issued a more controversial edict prohibiting lethal reprisals against former officials of the Baathist regime. "People even respected that, at least for a while," one Shia politician said.

Even as Saddam's statues were toppling in Baghdad, the BBC World Service reported (erroneously) that Ayatollah al-Sistani's modest house in Najaf was under threat from a hostile mob. The news spread like wildfire.

"I was sleeping in a village near Basra that night," said the scientist. "Suddenly I saw the villagers grabbing guns and preparing to rush to Najaf, hundreds of miles away. 'Sistani is under attack', they told me. That was all they needed to know. The same thing happened all over Iraq."

One example of his ability to get results was his impact on Iraqi petrol queues this year. Fuel shortages had been exacerbated by black marketeers cornering supplies, leading to enormous queues at petrol stations.

Finally, Ayatollah al-Sistani issued a fatwa against black market profiteering in petrol. The lines shrank by 75 per cent.

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