British troops are to be pulled out of the Sangin area of Afghanistan, where nearly a third of UK deaths in the war have occurred.
The Government has decided that the notoriously volatile district in northern Helmand province will be handed over to US forces, possibly later this year.
The move - to be announced today by Defence Secretary Liam Fox - comes after Britain handed over command in Helmand to an American general last month. Mr Fox is expected to say that Britain will concentrate on Helmand’s populous central belt, leaving the north and south of the province to the United States.
Sangin is particularly dangerous because it contains a patchwork of rival tribes and is a major centre of Afghanistan’s opium-growing industry. It has been the scene of some of the fiercest fighting the British military has endured since the Second World War.
Of the 312 UK deaths in Afghanistan since 2001, 99 occurred in Sangin.
Britain’s 8,000 forces in Helmand are now greatly outnumbered by the 20,000 US troops sent there under President Barack Obama’s surge strategy.
The Government last night faced warnings that the move would be exploited by the Taliban to suggest a retreat by the British.
Nato commanders had discussed the possibility of moving the UK’s 8,000 forces in Helmand to neighbouring Kandahar Province when Canada pulls out its contingent there next year.
But Mr Fox said last month it was "highly unlikely" this would happen, adding: "It is certainly not something that we will be proposing."