A Royal Marine charged with murdering a prisoner in Afghanistan told a court martial today that he thought the man was already dead when he shot him in the chest at point-blank range.

The sergeant said the shooting was the result of “stupidity, lack of self control, lack of judgment” and he was “ashamed and embarrassed” by what he had done. He accepted that he had lied to the police about the death, but insisted that this was because he feared that his actions could have been “misconstrued”.

The prosecution claims that a patrol of marines killed the man, a Taliban fighter, after they had found him lying wounded and helpless in a field in Helmand.

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Footage of the “battlefield execution” was recorded by a camera mounted on the helmet of one of the patrol and shown to the military tribunal in Bulford, Wiltshire, last week. The defendants, known in court as Marines A, B and C, of 42 Commando Brigade, cannot be named for legal reasons.

The sergeant, Marine A, stated that he thought the man, who had “fist-sized hole” in his back, had already died from injuries inflicted in a strike from an Apache helicopter. “It was my belief at the time I discharged my pistol that he was dead and no one had killed him,” he said.

When asked about video appearing to show the detainee, a young man, moving his arm and leg and convulsing after being shot, Marine A said he was “surprised” because the prisoner “suddenly became quite animated. I questioned whether I had made a mistake,” he added. Marine A told the court that his “stress and anger” had taken over when he pulled the trigger.

Prosecutor David Perry QC put it to him that the video did not show him to be under stress, or lacking self-control. He said: “You were calm and collected ... You leant into him, you aimed at a precise point didn’t you? Why did you do that?” The sergeant responded: “I don’t know.”

Marine A was then asked about a scene from the video, in which he says: “There you are, shuffle off this mortal coil you c***.” The sergeant told the court: “I believe it came from one of Shakespeare’s plays. It was ... bravado, something I am not proud of.”

When asked about another quote from the video in which he says: “I have just broken the Geneva Convention”, he said his concern related to strictures over the treatment of dead bodies, rather than any fear that he had taken a life. Marine A admitted under cross-examination by Mr Perry that he had not checked the prisoner’s pulse or breathing to ascertain whether he was dead or alive, either before or after the shooting.

The case continues.