Three Royal Marines carried out the "execution" of a suspected insurgent as he lay badly wounded after being hit by helicopter cannon fire in Afghanistan, a court martial was told on Wednesday.

Footage of the helpless, bloodied man being dragged across a field and the moment a sergeant bends down and apparently shoots him in the chest at close range was shown in court.

The sergeant, who can be identified only as Marine A, is allegedly heard telling the man: "There you are, shuffle off this mortal coil, you cunt. It's nothing you wouldn't do to us."

A few moments later Marine A is allegedly heard telling colleagues: "Obviously this doesn't go anywhere fellas. I've just broken the Geneva convention."

The footage was captured on a camera fixed to the helmet of another of the men, Marine B, who is accused of helping Marine A carry out the murder.

A third marine, identified as C, later allegedly wrote in his journal that he had wanted to shoot the captive in the head and had been upset that he had not fired the fatal round. "I felt no pity for him; fucker had been shooting up our boys," he is said to have written.

David Perry QC, prosecuting, told the military court in Bulford, Wiltshire, that the prisoner posed no threat. He said: "This killing was not in the heat of armed conflict. It amounted to an execution, a field execution, the execution of a man entitled to be treated with dignity and respect."

Perry said the identity, age and nationality of the victim was not known.

"What is known is that he was a detained person. At the time of the killing he was under the control and in the custody of the defendants."

While Marine A had allegedly fired the fatal shot, Perry said B and C had "encouraged and assisted" him.

All three deny murder.

The alleged killing took place in September 2011 at a time when the marines were suffering devastating losses and under huge pressure.

On the afternoon of the alleged murder, a command post in Helmand came under small-arms fire from insurgents and an Apache helicopter was called in. It located a suspected insurgent and fired more than 100 rounds at him, leaving him for dead. Perry told the court that the helicopter assault was an "ordinary legitimate incident of the conflict in Afghanistan."

A group commanded by Marine A, an Iraq veteran considered to be a safe pair of hands by his seniors, was given the task of carrying out a "battle-damage assessment" to check the impact of the helicopter attack. Perry claimed the men found the suspected insurgent badly injured, but still alive.

Perry told the military court that A ordered the man to be moved out of sight of a British observation balloon and the men pretended to treat him while the Apache hovered nearby. But once the helicopter left, Marine A allegedly bent down and shot the man at close range with a 9mm pistol.

The alleged murder only came to light a year later when footage from B's head camera was found on the laptop of another marine during an unrelated investigation. A claimed he had thought the man was already dead when he fired at him. He said it was a "stressful time" and he had taken out his anger on the man's body.

Perry said a journal had been discovered allegedly written by Marine C in which he reported that he had asked A if he could shoot him in the head.

According to the journal, Marine A replied it would be too obvious. The prosecution alleges C wrote: "I was ready and waiting to pop him one with my 9mm. I waited for the nod from A."

He continued: "A came over and popped him one himself. A squared it over the radio and sent it up, that he had snuffed it while we were treating him. I felt mugged off but job done, little fucker dead at the end of the day."

The three marines appeared in court amid tight security, hidden from the media and public behind a screen. The board hearing the case was told they had been given anonymity because of security issues.

The court martial continues.

The video footage

The video shot on a camera set up on Marine B's helmet was played to a hushed courtroom.

It showed the Apache helicopter flying in and opening fire. An unidentified voice shouts: "Shoot the cunts!"

Marines including A, B and C go to check if the Apache has killed a suspected insurgent, but they find that he is still alive.

A is allegedly heard saying: "Anyone want to do first aid on this idiot?" B allegedly replies: "No."

C allegedly says: "I'll put one in his head if you want."

There is laughter, then a voice – allegedly A – replies: "No, not in his head, 'cos that will be fucking obvious."

The prisoner is dragged through a cornfield. His shirt and trousers are bloodstained but he appears to still be alive and moving. He is thrown to the ground.

An unidentified voice says: "Just strangle him." Marine A allegedly reports to base that the man is dead. At this point the man appears to move and C nudges him with his foot before allegedly saying: "Maybe we should pump one in his heart."

The helicopter leaves the area and the man is rolled on to his front. A allegedly shoots him the chest with his pistol. The man's body can be seen twitching and the prosecution claims the following dialogue takes place.

A: "There you are, shuffle off this mortal coil, you cunt. It's nothing you wouldn't do to us."

B: "I know."

A: "Obviously this doesn't go anywhere fellas. I've just broken the Geneva convention."

B: "Yeah, rog."

A [on radio]: "He's fully dead now."

Radio operator: "Yeah, roger."

Laughter is heard.