As part of a restructuring plan, command and control in southern Afghanistan will be split into two areas, according to the Ministry of Defence.
Command will be rotated between US and UK forces under the new structure.
The first commander in Helmand will be Major General Richard Mills of the US Marine Corps.
Under the arrangements, expected to be implemented by the end of the summer, the existing Regional Command (South) will be split into two new headquarters based in Helmand and Kandahar.
There will be a new Regional Command (South West) based in Helmand and the first commander will be Maj Gen Mills. It has been agreed in principle that this rotational command will be shared between US and UK forces.
The existing Regional Command (South), headquartered in Kandahar, is under the command of British Major General Nick Carter.
Also under the changes, the command of the 1,100-strong British battle group based in Sangin, scene of some of the bloodiest fighting involving British troops, and Kajaki will transfer to a US-led force from June 1.
The changes were announced by Major General Gordon Messenger at a press briefing at MoD headquarters in central London.
He said: "This command and control change makes complete sense and is welcome. The span and complexity of the command challenge in southern Afghanistan has increased enormously in recent months and these changes provide the best command support to the troops on the ground."
The new strategy follows negotiations between the US, Britain and the other main contributors to the Isaf (International Security Assistance Force) in Afghanistan.
It has proved sensitive in Britain and until now UK troops answered to Maj Gen Carter, who was in charge of the whole of southern Afghanistan.
Maj Gen Messenger said that when Maj Gen Carter left his post in November as commander of Regional Command (South) the "likelihood" was that his replacement would be an American Army two-star commander.
Asked if the announcement meant Britain was effectively giving up its responsibilities and handing them over to the Americans, Maj Gen Messenger said: "That is simply not the case, this is us doing exactly the same job as we have been doing up to now, under slightly different arrangements.
"The whole business of a greater American presence is a reflection of the scale of the challenges that the British have faced in Helmand.
"I think we should take considerable pride of the families and the soldiers themselves should take enormous pride in the fact that for several years they were facing that challenge without this level of force."
