Seven Royal Marines have been arrested on suspicion of murder following an incident in Afghanistan last year, the Ministry of Defence said.
The Royal Military Police made the arrests in connection with an incident which occurred after "an engagement with an insurgent", the MoD said.
No civilians were involved and an investigation has been launched by the Service Justice System.
It is thought the arrests were of Royal Marines from 3 Commando Brigade, the BBC said, who were based in Helmand province last year.
An MoD spokesman said: "The Royal Military Police have today arrested seven Royal Marines on suspicion of murder. The arrests relate to an incident in Afghanistan in 2011.
"The incident followed an engagement with an insurgent; there were no civilians involved. The investigation will now be taken forward and dealt with by the Service Justice System.
"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."
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.