About 1,000 soldiers will be pulled out of Sangin district in the north of Helmand province, and replaced by some of the 20,000 US marines in the province, according to BBC reports.
Of 312 British service personnel to have died in Afghanistan since operations began there in 2001, about a third were killed in Sangin.
Defense Secretary Liam Fox was expected to announce later Wednesday the forces will refocus their efforts on Helmand's central belt.
The BBC also reported extra troops will be brought from Cyprus for security and logistics to aid with the Sangin withdrawal.
The decision follows Britain handing over command in Helmand to a US general last month.
While policymakers will strive to present the changes as simply a reorganisation of international forces in the province, observers voiced fears the move could be seen as a retreat and used as propaganda by the Taliban.
"People will assume," said former Liberal Democrat leader Menzies Campbell, "that this is preparing the ground for the eventual withdrawal in 2015. "It is bound, of course, to be interpreted in that way by the Taliban."
He added the move will recall Basra, Iraq, "where the moving of British troops was somehow presented as if they had created some kind of retreat".
Sangin is particularly dangerous, because it contains a number of rival tribes, and is a major centre of the country's opium trade.