The soldier was part of the Brigade Advisory Group, conducting a routine framework patrol with members of the Afghan National Army. He was in a fire support position, tracking the progress of a foot patrol from his Jackal vehicle. The foot patrol was returning to the vehicle when both teams came under small arms fire.
Spokesman for Task Force Helmand, Lieutenant Colonel Tim Purbrick, said:
"It is with great sadness that I have to announce the death of a soldier from the 9th/12th Royal Lancers (Prince of Wales's).
"The soldier was on a routine partnered patrol with the Afghan National Army in the Nahr-e Saraj district of Helmand province, to engage with the local population, when the patrol came under small arms fire. The soldier received a fatal gunshot wound during this incident.
"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 Forces investigation.
"Our thoughts and prayers are with the soldier's family and friends."
Next of kin have asked for a period of grace before any more details are released.