British troops in Afghanistan are to hand over responsibility for the Sangin area of Helmand province to US forces, the BBC understands.
The UK has suffered its heaviest losses in the area with 99 deaths since 2001.
British Defence Secretary Liam Fox is expected to tell MPs the move could happen by the end of the year.
The military insists the move is a redeployment now there are more US troops on the ground, but the Taliban are certain to portray it as a defeat.
Difficult questions
Last month Britain handed over command in Helmand to a US general.
Maj Gen Richard Mills, of the US Marine Corps, assumed control of all Nato-led International Security Assistance Force (Isaf) troops in Helmand on 1 June.
Mr Fox is expected to announce Britain will concentrate on Helmand's busy central belt, leaving the north and south to the US.
The BBC also understands that more British troops will be sent to Helmand to help with the redeployment.
The logistic and security troops will come from the Theatre Reserve Battalion stationed at Episkopi in Cyprus, according to military sources.
The Theatre Reserve Battalion for Afghanistan is currently provided by the 2nd Battalion, The Duke of Lancaster's Regiment.
The BBC's Quentin Sommerville in Kabul said the the deployment of the additional British troops to the area is intended to be purely temporary, while existing UK troops are moved from Sangin.
BBC defence correspondent Jonathan Beale said Mr Fox is likely to face some difficult questions, not least from the families of the British servicemen killed in Sangin.
'Bail out'
He said: "They may well ask... 'why have we sacrificed so much just to hand over responsibility to the Americans'?".
The Ministry of Defence will also be concerned it will be seen as a pull out or withdrawal by the British, with US forces bailing out UK troops, our correspondent added.
He said Mr Fox, backed by commanders on the ground, is likely to argue the transfer makes military and strategic sense as Sangin is now under US command and there are more US troops on the ground.
He is also expected to tell MPs Britain is not lowering its profile or commitment to Afghanistan.
But Conservative MP and former British army officer, Patrick Mercer, said the handover was a routine move and should under no circumstances be considered a retreat.
He told the BBC: "It's absolutely straightforward and normal in coalition warfare like this for units to serve under foreign command, but it's not necessarily desirable.
"There are now enough troops for this no longer to be necessary and any suggestion that British forces are being beaten out of Sangin or returning with their tails between their legs is not just disingenuous, it's actually disgusting."
Of the 312 UK deaths in Afghanistan since 2001, a third have taken place in Sangin, currently home to 40 Commando Royal Marines.
Col Stuart Tootal, who commanded the first UK battle group of of 1,200 soldiers sent into Sangin four years ago, told the BBC's Today programme he agreed the decision was a "logical military decision".
He said the number of deaths that had taken place in Sangin meant there was a lot of "emotion" attached to the area.
He said: "This decision is likely to have been taken as the US controls the area where Sangin sits in. They are bringing more troops in and the British main effort has moved over the last two months to the centre of Helmand.
"It makes no sense for a logistic and command point of view to keep a British battle group away from its main brigade when it's now an American area and there are American troops to take over from them.
"This reflects good practical military sense and we shouldn't allow emotion or misinterpretation to be put above that."
Rival tribes
Col Tootal stressed the move was a practical handover which represented a realignment of the new Nato command structure in Afghanistan.
It is the latest part of the province to be handed over to US control after the town of Musa Qaleh in March and the Kajaki dam last month.
Sangin has witnessed some of the fiercest fighting the British military has endured since World War II, and contains a mix of rival tribes.
It is also a volatile northern district at the heart of the opium-growing industry.
The UK's 8,000 forces in Helmand are greatly outnumbered by the 20,000 US Marines sent there by President Barack Obama.
The news comes after the latest British casualty of the war in Afghanistan died in a UK hospital on Monday after being injured in an explosion in the Nahr-e Saraj district of Helmand the day before.
The soldier, from 1st Battalion, The Mercian Regiment, was on patrol when he was caught in the blast.
