Seven Royal Marines have been arrested on suspicion of murder following an incident in Afghanistan during a tour of duty they served last year, the Ministry of Defence (MoD) said.
The Royal Military Police arrested the soldiers, believed to be members of 3 Commando Brigade, in connection with an incident which occurred while they were based in Helmand last year.
The incident was described as "an engagement with an insurgent", with no civilians involved, and an investigation has been launched by the Service Justice System.
An MoD spokesman said: "These arrests demonstrate the department and the armed forces' determination to ensure UK personnel act in accordance with their rules of engagement and our standards. It would be inappropriate to make any further comment while the investigation is under way."
During a six-month tour of duty, which lasted from April to October last year, 3 Commando Brigade saw seven servicemen killed in action, all from 42 Commando.
The tour, Operation Herrick 14, was the unit's fourth and saw the force score notable successes in capturing explosives from the Taliban.
Brigadier Ed Davis, who commanded Task Force Helmand and led 3 Commando Brigade during the tour, said his men had "interdicted" more than 2.5 tons of home-made explosive by August last year, leading to a 43% drop in enemy attacks.
The rules of engagement, largely derived from the Geneva Convention, dictate under what circumstances British troops are allowed to open fire, whether that is to prevent an attack by the enemy or in direct contact.
The arrests are thought to be the first time UK servicemen have been held on suspicion of such charges during the conflict in Afghanistan.
A Territorial Army soldier was investigated by military officials after shooting dead a suspected Taliban bomber in the Nahr-e-Seraj area of central Helmand in the summer of 2010. Senior officers believed the man may have been an innocent farmer but after an 18-month inquiry no charges were brought against Fusilier Duane Knott, who said he had no regrets over his action.