A soldier from 1st Battalion The Royal Irish Regiment has been killed by an explosion in Afghanistan, the Ministry of Defence says.
The soldier, from County Londonderry, died when an improvised explosive device detonated while he was on patrol in Nad-e Ali, Helmand province.
His death, on Remembrance Day as troops honoured fallen comrades, raises the toll in Afghanistan since 2001 to 344.
The soldier's family has been informed, the ministry said.
Spokesman for Task Force Helmand, Lt Col David Eastman, said: "On this day of all days, his loss feels especially poignant and, as with all of our brave, fallen comrades, will be forever marked in our thoughts; he will be sorely missed."
Northern Ireland First Minister Peter Robinson said he was deeply moved and saddened: "This death brings home the reality of war. Remembrance Sunday is not about the past, it's about the present.
"It's about the family in Northern Ireland who are now grieving the loss of their boy. I extend my deepest sympathies to them."
Prince William, in his second visit to Afghanistan, attended Sunday's Remembrance Day ceremony at Camp Bastion in Helmand Province, where he laid a wreath at the camp's memorial.
About 2,500 service personnel joined him for the event.