Defence Secretary Liam Fox is expected to tell MPs on July 7 the move could happen by the end of the year. Mr Fox is expected to announce Britain will concentrate on Helmand's busy central belt, leaving the north and south to the US.

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.

Almost a third of the 312 UK deaths in the war have happened in Sangin, a volatile northern district at the heart of the opium-growing industry.

Last month Britain handed over command in Helmand to a US general.