Hamid Karzai will next week announce the handover of the last British-controlled area of Helmand, opening the way for the possible return home next year of thousands of troops from Afghanistan.
Nahr-e Saraj district is likely to be named among 230 areas to begin transfer to Afghan control, British officials in Kabul believe.
The announcement, which may come as early as Sunday, will mean all three British-garrisoned districts will have entered the transition process allowing Afghan forces take over security duties from the Nato-led coalition.
The process will take 12 to 18 months, meaning Afghan soldiers and police are likely to take charge in the middle of next year.
Lashkar Gah and Nad-e Ali, the two other British districts, began transfer last year.
Britain has around 9,500 soldiers in Afghanistan and David Cameron has said only 500 will leave this year after to bowing to requests from commanders to slow the pace of withdrawal.
However the handover of the last British territory now paves the way for a far larger withdrawal in 2013, as troops switch to training and support missions rather than combat.
Britain will then maintain a far smaller force after the 2014 deadline for the whole country to be under Afghan control.
Though British commanders say security has improved greatly in Nahr-e Saraj in the past 12 months, the district continues to see fighting and British casualties.
Cpl Andrew Roberts and Pte Ratu Silibaravi, both aged 32, were killed during a mortar attack on Forward Operating Base Ouellette last week.
Britain has lost 18 troops this year in Helmand.
Next week's announcement by Mr Karzai will mark the third tranche of areas to begin transfer.
Ashraf Ghani Ahmadzai, the former finance minister now in charge of the handover, said: "In 2014 we will face a national survival test, but our enemies have to know that this nation is committed to fulfil this process."
"The third stage is a difficult stage. We don't want to tell lies to Afghanistan's people, but these difficulties are not ones that have no solution. The fourth and fifth stages will be more difficult" (Telegraph)