British troops will be withdrawn from the Sangin area of Afghanistan later this year, Defence Secretary Liam Fox has told MPs.
He said the UK had agreed to a redeployment which will see US Marines assume responsibility for the area.
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.
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.
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.
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."