The audio of the moment an injured Afghan insurgent was shot by a Royal Marine sergeant has been released by a military court.

Three marines, who can only be identified as A, B and C, have pleaded not guilty to murdering the unknown captured Afghan national on 15 September, 2011 contrary to Section 42 of the Armed Forces Act 2006.

The unknown man had been badly injured in a helicopter attack in Helmand Province. The marines had found him injured in a field, a military court in Bulford, Wiltshire heard.

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In audio clips identified as video clip four and five in the trial, the marines are heard dragging the injured insurgent across the field. Marine C is heard saying “I'll put one in his head, if you want.” Marine A replies: “That'll be too fucking obvious.”

Marine C is the heard saying: “Maybe we should pump one in his heart.”

Marine A is alleged to have shot the Afghan national in the chest at close range with a 9mm pistol before telling him: “There you are. Shuffle off this mortal coil you c***. It's nothing you wouldn't do to us.”

He then allegedly turned to his comrades and said: “Obviously this doesn't go anywhere fellas. I just broke the Geneva Convention.”

Marine B responds: “Yep, rog: If it ever comes to light, it’ll have been a warning shot.”

Marines B and C are accused of being “party to the killing” and “ encouraged and assisted” Marine A to commit the murder.

The incident was captured on the head camera of a helmet worn by Marine B. The video has not been released as the court believes it could prompt revenge attacks against British soldiers or civilians, but agreed to release the audio and a few stills from the footage. The images must not show the insurgent or him being shot.

Marine A said that he “genuinely believed” the insurgent was dead when he shot him and that he was “ashamed and embarrassed” of his actions. Marine B denied any involvement in the alleged murder and Marine C told the court he was moved off before the alleged offence occurred.

He said his diary entries were the “ramblings” of a “scared and angry man” which did not accurately reflect the events.

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The prosecution at the court martial alleges the shooting was effectively an execution.

David Perry QC, prosecuting, told the court martial: “It was not a killing in the heat and exercise of any armed conflict. The prosecution case is that it amounted to an execution, a field execution.

“An execution of a man who was entitled to be treated with dignity and respect and entitled to be treated as any British serviceman or servicewoman would be entitled to be treated in a similar situation.”

The court martial board retired today to consider its verdict.