British troops will be withdrawn from the Sangin area of Afghanistan later this year, Defence Secretary Liam Fox told MPs today.
He said the UK had agreed to a redeployment which will see US Marines assume responsibility for the area where nearly a third of UK deaths in the conflict have occurred.
The UK will concentrate its effort on the central part of Helmand province instead, in what he said would provide "more manpower and greater focus" on that area.
Dr Fox also said 300 extra British troops from a reserve battalion kept on stand-by in Cyprus would be deployed on a temporary basis until the switchover was complete - likely to be in October.
"The result will be a coherent and equitable division of the main populated areas of Helmand between three brigade-sized forces, with the US in the north and the south, and the UK-led Task Force Helmand, alongside our outstanding Danish and Estonian allies, in the central population belt," he told the Commons.
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.
The move comes after Britain handed over command in Helmand to an American general last month.
Although the withdrawal of British forces has been presented as a purely military decision, there have been warnings that it will be portrayed as a defeat by the Taliban for propaganda purposes.
Prime Minister David Cameron told MPs that 2010 was the "key year" for the mission in Afghanistan and said it was time to "surge up" the military and political pressure.
Repeating his desire to see UK troops home within five years, he said: "We have set out very clearly what we want to achieve in Afghanistan. This is the key year where we surge up the military forces, we surge up political pressure.
"Let me be clear. Do I think that we should be there in a combat role or in significant numbers in five years' time? No, I don't.
"This is the time to get the job done and the plan we have envisages making sure that we wouldn't be in Afghanistan in 2015."
Dr Fox said the UK "fully supported" the changeover in Sangin, which it had been closely consulted on by the Nato-led International Security and Assistance Force (Isaf).
And he paid tribute to the progress made as a result of British sacrifices there.
"In Sangin, UK forces have made huge progress in the face of great adversity. The district centre has been transformed.
"Helmand as a whole is a safer place as a result of our endeavours and sacrifices there. I pay tribute to those who have lost their lives in Sangin and those who continue to serve there."
The extra troops - which will come from the 2nd Battalion, the Duke of Lancaster's Regiment - would provide "additional flexibility to reinforce progress" in central Helmand, he said.
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."
Senior military spokesman Major General Gordon Messenger said there would always be "a bit of Sangin" in the bloodstream of the Army and the Royal Marines.
But he said this should not outweigh Nato's "sensible" decision to redeploy military resources in southern Afghanistan.
"I accept that there is an attachment of the British Army and the Royal Marines to Sangin, which is born of spilt blood, a great deal of endeavour and some pretty tough sacrifices that have been made," he said.
"There will always be a bit of Sangin in the bloodstream of the British Army and the Royal Marines.
"What we can't do is allow that sentiment to outweigh what is sensible military reapportionment of resources to ensure that the capabilities that we have are best directed to achieve the mission's success."
A spokesman for the Stop The War Coalition said the news was a tacit admission of the "failure" of the war in Afghanistan.
"Over 100 British soldiers have died there - a third of the total deaths over nearly nine years.
"Many people will be asking what exactly they died for.
"There is no clear justification for the war, and much-trumpeted successes, such as the one earlier this year in Marjah, are eventually revealed to be failures.
"The British Government and Army will try to deny that it is a retreat, but it is hard to see it in any other way.
"Governments now increasingly know that they cannot win, but refuse to admit that withdrawal is the only route.
"As the war drags on, how many people will have to die as a result? How much more instability in the region will be caused by it continuing?"Reuse content