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.
"It would be inappropriate to make any further comment while the investigation is under way."
The BBC's Jonathan Beale said the military legal process was similar to any other criminal investigation, which meant the Royal Military Police can question servicemen for a set period of time - normally 24 hours - although it could be extended.
If charges are brought the case will be referred to the Director of Service Prosecutions, who will have to decide whether there is sufficient evidence to proceed to trial by court martial.
A court martial has similar powers to a Crown Court and could sentence anyone accused of murder up to life in prison.
The Afghan Defence Ministry's spokesman, General Zahir Azimi, welcomed the MoD's announcement and underlined the need for British and Nato forces to respect the laws of war.
Speaking to the BBC's Pashto Service he said: "One of the reasons for high numbers of civilian casualties is that foreign troops and others are careless."
Such incidents would go down in number, he added, if the law was "implemented" properly and "those involved in careless attitudes towards civilians face courts".
He said he was unaware of the further details about incident.
The armed forces' rules of engagement stipulate under what circumstances British forces are 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 faced making split-second decisions and knew they might be prosecuted if they made mistakes or acted beyond that remit.
The rules are very rarely made public, because 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 were hard to fight because of the ease with which enemy fighters could 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.
'Doing the right thing'
Professor Michael Clarke, director general of the Royal United Services Institute, said the case could have a serious impact on Britain's standing in Afghanistan.
He said: "There is no doubt about it, this is politically damaging because we're not talking about an individual case here.
"Last year, there was a case of a soldier shooting someone and a soldier stabbing someone - those are individual cases and you will get that just on the law of averages in combat zones.
"But this is seven people, a unit, a group of marines who are very highly trained."
Prof Clarke added that marines were generally regarded as an elite force who had a "very good record in Afghanistan".
"So the fact that this is seven people and it's an incident means it will cause some political embarrassment I think. So I guess the MoD is trying to keep control over the story, but I think they are obviously doing the right thing."
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.