LONDON, Oct 12: Seven Royal Marines have been arrested on suspicion of murder after one of their colleagues came forward and claimed an insurgent had been killed in a manner that broke the military’s strict rules of engagement.

The incident happened in Afghanistan’s Helmand province last year, but it is thought investigators only began an inquiry in recent weeks when the “code of silence” among the marines was broken by one of those who is said to have witnessed the alleged crime.

The seven marines were arrested on Thursday by the Royal Military police and the inquiry will now be undertaken by the Ministry of Defence’s service justice system, which was specifically set up to allow the investigation in the UK of personnel who may have committed an offence overseas.

The Ministry of Defence (MoD) said on Thursday night that it could not give details about the ongoing inquiry. But the disclosure is a blow to the Royal Marines, who are regarded as being among the elite of the armed forces, and are the pride of the Royal Navy.

It is a gift to the Taliban, which will undoubtedly seize on the allegation as evidence that Nato’s forces in Afghanistan cannot be trusted.

The Royal Marines were in Helmand last year between April and September as part of 3 Commando Brigade, which was led by Brigadier Ed Davis. They were operating in the districts of Nahr-i-Saraj and Nad Ali, where scores of British troops have died since 2006.

The MoD would not say when the alleged incident took place or where, but confirmed that it was alleged to have happened during a skirmish with insurgents.

Nothing untoward was reported to commanding officers at the time, but the investigation began about a month ago when an individual within the military came forward.

The MoD stressed that the alleged killing involved a combatant, not a civilian. This suggests the accused men are being investigated for behaviour after the insurgent was already in their custody.

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 MoD said “as with any serious incident of this nature, there will be an internal review to identify lessons learned. The nature of that review will reflect the developments in and, in due course, the outcome of the investigation”.

The Royal Marines won high praise for their conduct during Operation Herrick 14 - the name given to the deployment. When the marines returned in the autumn last year, the MoD said they had helped to “strangle the insurgency in Helmand”.

It was said their time in Afghanistan had been “historically significant”.

Brigadier Davis said the 6,500 strong brigade had mostly kept the insurgency in check, and that in the first four months of the tour alone, 16 low-level commanders had been killed or captured.

However, he warned that there was “a hard core of the insurgency left. It is becoming harder to kill or capture the ones who are left, so the challenge is still very much still there.”

Nineteen British service personnel died during the tour.

By arrangement with the Guardian