British forces in southern Afghanistan handed responsibility for security in Sangin to US forces, the Ministry of Defence said today.
The British Government first announced the transfer of authority in July.
Control of the area was handed from 40 Commando Royal Marines to the US Marine Corps shortly after 6.30am.
Defence Secretary, Dr Liam Fox, said: "British forces have served in Sangin over the last four years and should be very proud of the achievements they have made in one of the most challenging areas of Afghanistan."
He added: "The level of sacrifice has been high and we should never forget the many brave troops who have lost their lives in the pursuit of success in an international mission rooted firmly in our own national security in the UK."
UK forces have been in Sangin since 2006.
British troops will redeploy to central Helmand, where they will continue to lead the fight against the insurgency and assist in building a stable and secure Afghanistan, he said.
The number of British losses since the conflict in Afghanistan began in 2001 now stands at 337.
Ministry of Defence spokesman Major General Gordon Messenger, a former commander of the UK Helmand task force, insisted the handover was not an admission of defeat.
"It certainly won't look like that on the ground," he told the BBC Radio 4's Today programme.
"The British soldiers that are there are handing over to the American Marines. In terms of the physical security presence and every other aspect of the campaign in Sangin, it's going to be more of a continuum than a watershed."
He added: "We are seeing real and positive progress in areas that only a year or so ago were in a very different state."