A Royal Marine has been shot dead in Helmand province, Afghanistan - the fourth member of the British armed forces to be killed in one day.
The marine, from 40 Commando, serving as part of Combined Force Sangin, was killed while on foot patrol in Sangin on Tuesday. Family have been told.
Lt Col James Carr-Smith said the marine had made the ultimate sacrifice.
Meanwhile a hunt is under way for a rogue Afghan soldier who killed three UK soldiers in Nahr-e Saraj, Helmand.
Speaking about the marine's death, Lt Col Carr-Smith, spokesman for Task Force Helmand, said: "[The soldier] was part of a foot patrol - together with his fellow marines and Afghan National Army partners - when he was killed by small arms fire.
"He died in the course of his duty. He was a brave and courageous marine who made the ultimate sacrifice.
"He will be greatly missed and his actions will not be forgotten."
The four deaths on Tuesday take the number of British service personnel killed since operations began in Afghanistan in 2001 to 318.
Three soldiers were killed and four others injured by the renegade Afghan soldier on Tuesday morning.
One of the dead is believed to have been a Nepalese Gurkha and the other two were UK nationals.
The men, from 1st Battalion The Royal Gurkha Rifles, were on duty in Nahr-e Saraj, Helmand province, when the attack happened.
The BBC understands that one of the soldiers killed was shot and the other two were killed by a rocket-propelled grenade.
The Taliban have claimed the Afghan soldier had joined their insurgency.
Lt Col Carr-Smith blamed a "lone individual" who had betrayed his comrades.
"His whereabouts are currently unknown but we are making strenuous efforts to find him," he said.
"He should know that his actions will not deter us from our task and we will continue to work closely with our Afghan friends to bring security to Helmand."
Prime Minister David Cameron condemned the "appalling" killings, but also stressed UK and Afghan troops should continue to work together.