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 UK 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 US.
Sangin, currently home to 40 Commando Royal Marines, 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.
There is likely to be a mixed response to news of the handover – relief that such a deadly burden has been passed on but also sadness at the high price paid in troops killed and injured.
Britain’s 8,000 forces in Helmand are now are now greatly outnumbered by the 20,000 US Marines sent there under President Barack Obama’s surge strategy.
The UK government last night faced warnings that the move would be exploited by the Taliban to suggest a retreat by the British.
Tory MP and former soldier Patrick Mercer said the move was based on “sound military logic” but accepted it presented a propaganda opportunity.
The end of this month marked the 130th anniversary of a bloody defeat for British and Indian forces at Maiwand on the Helmand river, he noted.
“If we are not careful our enemies will big up this and make it look like a defeat when it isn’t; it is simple military logic,” Mr Mercer told BBC2’s Newsnight.
“The perception of this is all important.”
Former Liberal Democrat leader Menzies Campbell said: “The political context of course has got to be what the Prime Minister said following his visit to Toronto and the G20 – namely that he expected British troops to be out by 2015.
“People will assume from this that this is preparing the ground for the eventual withdrawal in 2015 and it is bound, of course, to be interpreted in that way by the Taliban.
“At our peril, we fail to understand and estimate the sophistication of these people and their ability to turn facts into propaganda.”
He said the “very unusual” decision to announce such a military shift in the Commons underlined the political sensitivities – and suggested the Taliban would also draw parallels with Iraq.
“This is a replacement not a reinforcement and of course people will be casting their minds back to Basra, where the moving of British troops was somehow presented as if they had created some kind of retreat,” he said.
“I’m sure that will be denied and I am sure there are very sound military reasons for doing what we are doing. But it undoubtedly has political implications and that explains why Liam Fox believes it is necessary to come to the House of Commons.”
Sangin is the latest part of the district to be handed over from British to American control.
The town of Musa Qaleh was transferred in March and US marines took charge of the strategically important Kajaki dam last month.
Major General Richard Mills, of the US Marine Corps, assumed control of all Nato-led International Security Assistance Force (Isaf) troops in Helmand on June 1 as part of a reorganisation of military command structures.
Nato commanders also 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 Dr Fox said last month it was “highly unlikely” this would happen, adding: “It is certainly not something that we will be proposing.”
The Ministry of Defence refused to comment on today’s expected announcement.
An MoD spokesman said: “UK forces continue to make real progress across Helmand, including in Sangin, one of the most contested and challenging areas in southern Afghanistan.
“Isaf is responsible for ensuring the most effective allocation of international forces to deliver the campaign strategy in Afghanistan and the UK fully supports Isaf commanders in this aim.
“Any changes to force laydown affecting UK personnel will be announced in the usual way.”