Plans to bring home 1,600 troops from Iraq this spring are in disarray, Ministry of Defence officials said last night.

The admission came as the Iraqi Government’s offensive against Shia militias in Basra appeared to be failing.

The rebels ignored a deadline to disarm and intense fighting in the city raised the possibility that British forces could be asked to re-engage on the front line.

Nouri al-Maliki, the Iraqi Prime Minister, had flown to Saddam Hussein’s former palace in central Basra to take personal control of the offensive, led by 30,000 Iraqi troops backed by paramilitary police.

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But reports from the city suggested that the Iraqi forces had failed to make any significant inroads. As the deadline for disarmament passed, Iraqi police were defecting to the militia ranks.

The Iraqi Government’s difficulties leave Britain facing one of its toughest challenges since the invasion five years ago.

MoD officials admitted that they were no longer thinking about cutting troop numbers to 2,500 from the spring, as had been outlined by Gordon Brown in a statement to the Commons last October. “Any plans for a reduction of British troops is off the table for the time being,” a senior Whitehall source said.

Officials insisted that so far British forces were providing support only in the form of air cover and logistics support — including 19 liaison officers in Basra city — but did not rule out sending a small force to help the Iraqis if requested by the authorities in Baghdad.

Mr al-Maliki had hoped to lead his army to victory in Shia militia strongholds in Basra, Iraq’s oil city in the south. Instead, Iraq’s Shia prime minister was left with the prospect of disaster as district after district of his own capital fell to the rival Mahdi Army.

Residents of Basra complained that water and electricity had been turned off in the three main areas besieged by the Iraqi Army. Estimates of the death toll in Basra are as high as 200, with hundreds more wounded.

Under present strategy, the 4,100 British troops still in Iraq are supposed to remain at their base at the airport northwest of Basra and join the Iraqi forces in a security operation only when requested.

No such request has been made and a source in Basra said there was not expected to be any call for help during the present operation.

President Bush praised the high-risk strategies. He said: “Prime Minister Maliki’s bold decision to go after the illegal groups in Basra shows his leadership and his commitment to enforce the law in an evenhanded manner.”

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