The Oxford Research Group non-governmental organisation, which assesses constructive approaches to dealing with international terrorism and the "war on terror", says the war in Iraq is only in its early stages.

"Given that the al-Qaeda movement and its affiliates are seeking to achieve their aims over a period of decades rather than years, the probability is that, short of major political changes in the USA, the Iraq war might well be measured over a similar time span," the report concludes.

It says the presence of coalition troops in Iraq since the March 2003 US-led invasion has been a "gift" to Osama bin Laden's Al-Qaeda.

The terror network has gained recruits by portraying their presence as a neo-Christian occupation of a main Muslim country, the report says.

The group says an American pullout would be "a foreign policy disaster greater than the retreat from Vietnam".

There was no prospect of British troops coming home from the country unless there was an about-turn on Britain's relationship with the US.

Prime Minister Tony Blair has been a staunch supporter of action in Iraq.

Defence Secretary John Reid said last week that British troops could start withdrawing next year.

The report says: "This would be a major policy shift for the Blair Government, representing the sharpest difference in its relationship with Washington in the past eight years.

"In present circumstances it is highly unlikely, yet the war is likely to cast an increasing shadow over UK policy in the next year."

Assuring Iraq's security and the presence of a friendly government in the country is an essential strand of American security policy, even if it meant keeping a permanent military presence in Iraq, the report says.

That would allow the US long-term access to oil from the region, essential to the US because of its increasing dependence on foreign oil, it says.

The Oxford Research Group said in July that nearly 25,000 civilians had died in violence since the start of the war in Iraq, a third of which were killed by coalition forces.