Seven Royal Marines have been arrested on suspicion of murder, the Ministry of Defence has said.
The arrests by the Royal Military Police related to an incident in Afghanistan last year, when 3 Commando Brigade was based in Helmand.
The MoD said the incident followed an "engagement with an insurgent" and no civilians were involved.
It is thought it is the first time UK servicemen have been arrested on such charges during the conflict.
An MoD spokesman said: "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.
The rules of engagement stipulate under what circumstances British forces were allowed to open fire, either in direct contact with the enemy or to prevent an imminent attack.
BBC defence correspondent Caroline Wyatt said troops often face making split-second decisions and know they may be prosecuted if they make mistakes or act beyond that remit.
The rules are very rarely made public, as the armed forces do not want insurgents to know more than they already do about the circumstances under which UK service personnel are permitted to fire on them or respond with force.
Insurgencies are hard to fight because of the ease with which enemy fighters can drop their weapons and switch in an instant from armed insurgent to blending in with the civilian population, our correspondent said.
The MoD said that as with any serious incident of this nature, there would be an internal review to identify lessons learned.
Some 23 servicemen from 3 Commando Brigade lost their lives in 2011 during a summer tour of duty that encompassed the height of the fighting season, while many more were injured.
Altogether, they conducted 41,000 patrols after taking over command of Task Force Helmand in April that year - the fourth time that 3 Commando Brigade had deployed on Operation Herrick in Afghanistan.
The MoD is now likely to be liaising with 4 Mechanized Brigade, the UK force now in command of Task Force Helmand, to ensure that all necessary force protection measures are taken in case there is a reaction in Helmand to the news of the arrests on suspicion of murder.
Defence sources are keen to stress that the vast majority of UK forces in Helmand have conducted themselves according to the standards expected of them, our correspondent added.
However, with tensions still high after the recent anti-Islam video released in the USA, troops currently serving in Afghanistan will be on alert, as the Taliban could use the arrests to try to bolster support for the insurgency.