The plan, which has been submitted to Donald Rumsfeld, the US defence secretary, calls for more than 60,000 troops — over a third of the total — to leave by the end of next year.

According to US television reports this weekend, the total number left in Iraq would fall from 161,000 to what defence experts call the “magic number” of below 100,000.

The Pentagon refused to confirm the proposals last night but a senior Ministry of Defence source said they appeared to “match our own plans” for a phased withdrawal under which most British troops will have left Iraq by the end of 2006.

Officials expect UK numbers to be cut by 5,000 from the current level of close to 8,500.

President George W Bush said in South Korea yesterday that American forces would “stay in the fight until we have achieved the victory our brave troops have fought for”. But he added that strategy would be “driven by the sober judgment of our military commanders on the ground”.

General George Casey, the US commander in Iraq, told Congress in September that the large US military presence was fuelling the insurgency. It “feeds the notion of occupation”, he said, and “extends the amount of time it will take for Iraqi security forces to become self-reliant”.

The plan he has drawn up with General John Abizaid, commander of US forces in the region, envisages the number of troops falling to 138,000 soon after the December 15 election — a drop of 23,000.

One or two battalions — about 2,000 troops at a time — could then begin pulling out in January if the elections to form a new government prove successful. The withdrawal would continue throughout the year, beginning slowly.

The plan was intended to remain secret until after the voting out of concern that Iraqis would think America was preparing to cut and run. But Pentagon and military officials decided to go public after a week of political infighting.

Senate Republicans passed a resolution last week demanding that 2006 should be a “period of significant transition to full sovereignty” in Iraq. Party chiefs also want a substantial pull-out before mid-term congressional elections next November.

Rumsfeld has yet to approve the plan and will not do so until after the Iraqi elections, according to the Pentagon.

The proposals are hedged with warnings about ground conditions and the capabilities of Iraqi forces. Of 96 Iraqi battalions, only one is deemed capable of operating without US military support.

However, British officials said the US proposals mirrored their own for a phased withdrawal in 2006. British forces would retreat into five or six bases in the southeast, leaving the Iraqi security forces to control the ground. They would leave their bases only if the Iraqis lost control.

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