A British soldier from 9th/12th Royal Lancers (Prince of Wales's) has been killed in Afghanistan, the Ministry of Defence has said.
The soldier, who was in the Brigade Advisory Group, died on Saturday. His next of kin have been informed.
He was on a patrol with Afghan soldiers in the Nahr-e Saraj district of Helmand Province when his team was fired on.
A report that the fatal gunshot was fired by an Afghan National Army soldier is being investigated.
It is not yet clear whether the Afghan man was a genuine member of the army or an insurgent posing as a soldier.
The British soldier, who has not yet been named, was tracking the progress of a foot patrol from his vehicle. The foot patrol was returning to the vehicle when both teams came under small arms fire.
The incident comes as the Nato-led International Security Assistance Force (Isaf) prepares to hand over control of some security operations to Afghan troops.
Task Force Helmand spokesman Lt Col Tim Purbrick, said: "A report that the fatal gunshot was fired by an Afghan National Army soldier is now the subject of a joint International Security Assistance Force and Afghan National Security Force investigation.
"Our thoughts and prayers are with the soldier's family and friends."
The Afghan army has strict security screening procedures for its recruits, but there have been several incidents in which its personnel have fired on foreign soldiers.