Britain's military is to be handed a significantly diminished role on the front line in southern Afghanistan when President Obama completes a strategic review of a war that US officials say is in danger of being lost.

Mr Obama is preparing to make his first key deployment as US Commander-in-Chief over the next few days, with reports suggesting that he may send between 3,500 and 7,000 additional troops to Afghanistan.

The Americans are to build two huge new bases in the south. One will be one on the Helmand border with Kandahar at Maiwand - a place famous as the site of the destruction of a British army during the Second Afghan War of 1881. The other will be in Zabul, a province now largely controlled by the Taleban and criminal gangs.

Commanders have requested a surge of up to 30,000 soldiers, but the President is said to be carefully weighing up his options and the capacity of Nato allies before making a decision next month.

As The Times reported yesterday, although the US had hoped for at least 1,500 more British troops, the contribution could be limited to 300 on top of 8,300 already in Afghanistan.

Most of these are in Helmand and their performance has come in for sustained criticism from the Pentagon, where officials say British Forces have only lately begun to adopt the counter-insurgency techniques pioneered by General David Petraeus in Iraq. US forces ceded control of the south to Nato in 2006. Britain took responsibility for Helmand, Canada was to oversee Kandahar and Dutch forces moved into Uruzgan.

The subsequent two years have seen a relentless deterioration of security.

A US Aviation Brigade incorporating more than 100 transport and attack helicopters is earmarked for the south - a five-fold increase from the 20 or so helicopters owned by British, Dutch and Canadian forces.

In Helmand, 3,500 US Marines who were temporarily deployed in March 2008, operating alongside British forces south of the town of Garmsir, have been made a permanent posting. Their area of operations has been expanded to include the town of Nawzad, previously a British-held area.

US forces have also been embedded with Afghan police units in Musa Qala, another previously British preserve.

Colonel Greg Julian, the chief spokesman for US forces in Afghanistan, said that the incoming American troops would have a “mobile role” that would mean them being sent to areas of intense Taleban activity.

Western diplomats say that US air power would allow American forces to achieve far more than the British, whose stock is low as a result of the shortcomings of the missions in Helmand and Iraq.

Sir Jock Stirrup, Chief of Defence Staff, admitted at the end of January that British commanders had been “smug and complacent” when troops first deployed to Helmand in 2006. The battle for Musa Qala in December 2007 is understood to have created “considerable friction” with American commanders, who blamed the British for withdrawing from the city and allowing it to become a haven for insurgents. Although Britain's Major-General Nick Carter is still expected to take titular charge of Regional Command South next month, there is talk of introducing alternative command structures for US troops beginning to flood into the region.

Senior British officials say that parts of the British force may be redeployed to “different geographic areas” in Afghanistan and that there will be more focus on specialist operations such as clearing roadside bombs.

One said: “We've been in difficult public relations territory for some time. Helmand is hard to run and we want to avoid the impression that we are solely responsible for it. A third of our troops are already deployed elsewhere and that may be something we see more of in the coming months.”

David Kilcullen, who until last month was special adviser on counter-insurgency for the US State Department, said: “I would be very surprised if the British get formally removed from being in charge of the south. But I would be less surprised if there were to be a new command structure with a headquarters interposed at a level between Kabul and Kandahar.

“Yes, some people have criticised British performance, but it's a backhanded compliment - Americans judge Brits largely by the same standards they apply to themselves.” He said the problem was that the US “has leapt ahead of all its allies in recent years, not just in terms of technology or conventional warfare, but recently, expertise in counter-insurgency”.

In evidence to the House of Commons Liaison Committee yesterday, Gordon Brown welcomed the two-month review of US policy on Afghanistan and Pakistan, saying “a new set of proposals” on dealing with the security problems would emerge at the Nato summit in April.

He said that he had spoken to Richard Holbrooke, Mr Obama's envoy to the region, about the adoption of guerrilla-style tactics by the Taleban and the need to stop terrorists slipping over the border from Pakistan.

“Our policy must be to work with the people and the tribes of Afghanistan to make sure that in addition to the military action we are taking, we are training their soldiers and police, we are making sure that there is a system of local government that is effective,” the Prime Minister added.

The White House announced this week that Bruce Riedel, a former CIA official, would head the review into strategy on Afghanistan. Last month he identified the most pressing needs as shoring up security around Kabul and the south - where international forces were increasingly stretched.

“For the last two years British, Canadian and Dutch troops have been fighting desperately to stabilise Kandahar, Helmand and Uruzgan provinces against a determined Taleban based across the border in Pakistan,” he said.

General Jim Jones, Mr Obama's national security adviser, also suggested last week that co-ordination of international efforts had been “spotty at best”. At a foreign policy conference in Munich, he signalled a shift towards a new strategy that would set out limited but attainable goals including good governance, police training and stemming the opium trade. He said: “Barack Obama is a pragmatist.”

Mr Kilcullen said that there would be less focus on building democracy and seeking to destroy the Taleban than on stabilisation. “In the current financial climate, that means working with tribes,” he said. “We have been charging around chasing the enemy and not doing enough to protect the population.”

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