Seven Royal Marines have been arrested on suspicion of murder following an incident Afghanistan in 2011, 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".
No civilians were involved and an investigation has now been launched by the Service Justice System. The incident occurred in Nad-e-Ali.
A Ministry of Defence spokesperson has told ITV News the Marines arrested are from 3 Commando.
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.
Robert Fox, the Evening Standard's Defence Correspondent has told ITV's Daybreak that the arrests are "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."
An MOD spokesperson has outlined the procedure that will take place following the arrest of the marines:
The Marines are held in custody under usual civilian custodial regulations. Therefore, if after 24 hours no charges are brought, the Royal Military Police, who are investigating and evidence gathering, will have to apply for an extension of custody.
The RMP report to the director of service prosecutions, which is part of the services prosecutions authority - the equivalent to the CPS. He is a QC, an independent body and accountable to the Attorney General.
If there is sufficient evidence they will be charged with murder.