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.
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.
Robert Fox, the Evening Standard's Defence Correspondent has told ITV's Daybreak that the arrest of seven Royal Marines is unprecedented.
He said: "This is new ground because, this is the first time since the deployment to Helmand that the suspicions of this kind have been raised so formally.
"The interesting thing about this one is that the evidence seems to have come from inside the Royal Marines."
He added: "The shades of violence in Helmand are far more complex than people realise."
- Relations between Western forces and Afghan civilians have been strained in the last year after soldiers burned copies of the Koran at a NATO base and a U.S. soldier was accused of killed 16 civilians in a rampage in March.
- A video also emerged depicting U.S. marines urinating on corpses.
- Relations between Western troops and Afghan forces have also deteriorated after a series of "insider" attacks against NATO coalition troops by Afghan soldiers or by militants wearing Afghan military uniform.
The Ministry of Defence confirmed there would be an internal review into the incident.
– MoD statement
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 underway.