A member of the Afghan army who acted as a prayer leader for troops has shot dead a British soldier in the year's first insider attack, according to officials in the region.
As many as six British troops were also injured in the shooting in Helmand Province. The Taliban claimed responsibility for the attack in a statement released shortly after details emerged, the BBC reported.
It is the latest such incident in a string of insider attacks that threatens to undermine the handover from international forces to local troops.
A member of the Afghan National Army in Helmand said the attacker - shot dead after opening fire - was from Laghman Province, in the east of the country and had joined the Afghan army a year ago.
"He was well-known as being religious and would lead prayers, acting as an imam," said the soldier, speaking on condition of anonymity.
The Ministry of Defence said a soldier with 28 Engineer Regiment, attached to 21 Engineer Regiment, was killed at Patrol Base Hazrat in the Nahr-e Saraj area. His family have been informed.
Major Laurence Roche, a spokesman for Task Force Helmand, said: "This is an extremely sad day for the Corps of Royal Engineers and everyone serving with Task Force Helmand. Our thoughts are with the soldier's family and friends at this time."
The attack on Monday evening came as Hamid Karzai, the Afghan president, flew to Washington for talks with President Barack Obama to plan for the long-term security of the country as international forces leave next year.
"An individual wearing an Afghan National Army uniform turned his weapon against International Security Assistance Force (Isaf) service members in southern Afghanistan yesterday, killing one," is how Isaf, the Nato force described the killing in a statement on Tuesday.
The Ministry of Defence said the attacker had initially turned his weapon on fellow ANA members, before turning his fire on Isaf soldiers.
Last year, more than 60 troops from the force were killed in so-called "green on blue" attacks, leading to concerns about whether a strategy of training local forces to take over security when Nato combat forces leave in 2014 could work.
The Taliban frequently claim such killings are the work of their agents who have infiltrated Afghan security forces. However, ISAF and Afghan officials say that many are due to personal grudges.
Such is the severity of the threat that international soldiers working alongside Afghan security forces are often watched over by "guardian angels", troops tasked with providing personal protection.
Colonel Mohammed Rasool Zazai, a spokesman for the Afghan National Army, said: "We don't have the full details yet but have launched a full investigation."
By Rob Crilly, Zubair Babarkarkhail