President Bush is expected to promise today that he will maintain US troop levels in Iraq at their existing levels almost until his last day in the White House.

In a speech being delivered to the National Defence University, he will announce that one battalion - numbering just 1,000 Marines - can come home as scheduled in November, with their replacements instead being sent to Afghanistan.

But he then intends to wait until his last month in office - January next year - to bring home an army brigade and support troops totalling about 8,000.

There are currently 146,000 US troops in Iraq and 33,000 in Afghanistan. Mr Bush is said to have resisted strong pressure from the Pentagon to switch more of America's military might from Iraq - which has seen notable security gains in the past year - to Afghanistan, where US and Nato forces are increasingly stretched against the resurgent Taleban and al-Qaeda.

Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, and other senior military figures, are said to have argued for a bigger withdrawal from Iraq. They believe that, with violence falling in the country by about 80 per cent since last year, there is a pressing need to redeploy troops to Afghanistan where commanders have repeatedly asked for additional forces.

But General David Petraeus, the top US commander in Iraq, resisted the pull-out, telling the White House that security improvements there remain fragile. He is reported to have wanted all 15 army brigades to remain in place until June next year. Already, the Pentagon has withdrawn five combat brigades from Iraq this year, returning troop levels to the numbers they were before last year's successful "surge" strategy.

The compromise plan means Mr Bush's successor in the White House wll be left with a big decision to make on future troop levels next year when several more combat brigades are scheduled to leave Iraq.

Mr Bush will leave the White House on January 20. Democratic presidential nominee Barack Obama has advocated pulling all US combat forces out of Iraq within 16 months of taking office. But John McCain, his Republican opponent who is close to Gen Petraeus, has said he would rely on the advice of US military commanders to determine the timing and pace of any troop reductions.

For all his waning power in Washington, Mr Bush has remained able to run the war on his terms and now appears determined to use the success of the surge to repair some of the damage wrought to his reputation by this deeply unpopular conflict.

Yesterday, White House spokesman, Dana Perino, said: "I think that the surge is no doubt one of the most important foreign policy and military decisions that have been made in a generation, and it was fundamental to the change that we have seen today in Iraq.

"We are working now to cement those gains and to be able to continue to watch Iraq evolve into a country that can sustain, govern and defend itself."

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