Patrick Cockburn: 'Troops are camped on the flanks of a volcano'

Margaret Beckett and Des Browne enter office having to pretend American and British forces in Iraq are not encamped on the flank of a volcano. The pretence makes it ever more likely that, when the explosion happens, the soldiers on the ground will suffer significant casualties.

The danger is very simple. For three years, the US - with Britain tagging meekly along behind - has been combating a rebellion by the five million Sunni Arabs in Iraq. During that time, the US has lost 20,000 of its solders, dead or injured. Britain, with far smaller forces stationed in the safer Shia deep south of Iraq, has had 109 soldiers killed.

Given that the US forces have barely been able to hold their own in a guerrilla war against the Sunni - only 20 per cent of the population - they cannot face a war against the 15-16 million Iraqis who are Shia. In 2003, the Shia were glad to see the end of Saddam Hussein. They did not resist the occupation as long as it did not prevent them taking power through elections which they, as 60 per cent of the population, were bound to win.

The political and military volcano rumbling under the feet of the US and Britain in Iraq is that an ever increasing number of Shia want them to go.

That was the message of the cheering crowds in Basra on Saturday celebrating the downing of the British helicopter. It should not have come as a surprise. Last September, the MoD absurdly claimed the crowds dancing in jubilation as British armoured vehicles burned were small and unrepresentative. The truth was the British army has become as unpopular in Basra as US soldiers have always been in Baghdad.

The policy of the Shia religious hierarchy, headed by Grand Ayatollah Ali al-Sistani, has been consistent. It did not support the occupation. Mr Sistani has always refused to meet its representatives. He insisted on elections despite equivocations by the US. The polls on 30 January and 15 December last year were won by the Shia religious parties. These are allied to the Kurds and together they form a government.

But the Shia leaders increasing see the US and Britain as playing a game of divide-and-rule by switching support to the Sunni. The US fully supported the ultimately successful and extraordinarily long campaign by the Kurds to get rid of Ibrahim al-Jaafari of the Shia Dawa party as prime minister. It may not do them much good since he has been replaced by his deputy Nuri al-Maliki.

The danger to the remaining British troops in southern Iraq is that they are not just facing some rogue police units infiltrated by Shia militia.

Crowds chanted "we are all soldiers of al-Sayed [Muqtada al-Sadr]" in Basra as they celebrated the shooting down of the helicopter. British and American officials speak of the infiltration of the police by the Mehdi Army of al-Sadr and the Badr Organisation of the Supreme Council for Islamic Revolution in Iraq (SCIRI). They do not mention when they can help it that those same militias are controlled by the Shia religious parties who form the government. The biggest militia of all is that of the Kurds who have 60,000 men under arms.

The deterioration of relations between British troops and the Shia of southern Iraq can be exaggerated. It was never as good as it seemed after a brief honeymoon after the overthrow of Saddam Hussein. In June 2003, I drove to Majar al-Kabir, where six British military policemen had just been murdered in the police station. I had a local bodyguard, cradling his sub-machine gun on his knee, who was meant to protect me from anti-British forces. But on the way back he confided that "we are just waiting for our religious leaders to issue a fatwa against the occupation and then we will fight".