A prisoner was murdered in cold blood in Afghanistan by a Royal Marines patrol after they had found him lying injured and helpless in a field, a court martial has heard.

Footage of the alleged “battlefield execution” of the Taliban fighter in Helmand two years ago was recorded by the helmet camera of one of the alleged killers and shown to the proceedings at a military tribunal in Bulford, Wiltshire.

Leaning to fire a bullet into the chest of the young Afghan covered in blood, a sergeant in charge of the group was heard to say: “There you are, shuffle off this mortal coil you c***, it’s nothing you wouldn’t have done to us.” Turning to other marines, he added: “Obviously this don’t go anywhere fellas, I’ve just broken the Geneva Convention.”

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The victim was left twitching and gasping for breath as the British servicemen, apprehensive that they might have been spotted, pretended to give him first aid. They then called in their base, saying the man had died from the injuries he had already suffered in a helicopter strike, the court was told.

There have been several high-profile prosecutions of British service personnel over killings of civilians in Iraq but this is the first case of UK personnel charged with murder in the course of the mission in Afghanistan.

The alleged killing came when the Taliban was carrying out indiscriminate bombings of both civilians and Western forces in Helmand, the court was told. The Royal Marines’ 42 Commando Brigade suffered seven fatalities and 42 injuries.

One of those who died was Lieutenant Oliver Augustin, 23. Body parts of other Marines were hung from a tree by the insurgents, the court was told. The killing of the Afghan detainee took place soon afterwards. Three members of the brigade – a sergeant, a lance corporal and a marine – face charges of murder on 15 September, 2011. Their identities cannot be made public under a court order.

The defendants, referred to as Marines A, B and C, pleaded not guilty from behind a blue screen shielding them from the view of the public. They could, however, be seen by Judge Advocate General Jeff Blackett and the military jury of seven.

The prosecution alleges that it was Marine A, an experienced sergeant, who had carried out the shooting. Marines B and C encouraged and assisted him. They were arrested after the footage was discovered on the laptop of another marine, who does not face any charges. Police subsequently found a diary kept by Marine C in which he claims he witnessed Marine A carry out the killing. His only regret, he wrote, was that he felt “mugged off” that he had not been able to “pop off the Taliban s***bag” himself.

After his arrest Marine C said he had written his diary when under stress and it did not reflect what really happened. “I played no part in this, I did not murder anyone,” he insisted.

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David Perry QC, prosecuting, said: “The video speaks for itself. It clearly shows they killed an injured man; it shows they lied about the circumstances in which the injured man met his death. 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 alleged murder took place following an attack on a command post by insurgents. An Apache helicopter-gunship carried out an intense strafing of the area. It was thought highly unlikely, said? Mr Perry, that anyone would survive such an onslaught. However, a patrol led by Marine A found a badly injured man lying in a corn field.

Marine A asked Marine C to move the man to a less exposed area. He was worried, claimed the prosecution, of being seen by the Apache, which was still hovering near by, as well as an observation balloon.

In the video, Marine A is heard to say “Get him close in, so the PGSS [observation balloon] can’t see what we’re doing to him”. He later asks: “Where’s the Ugly? He’s over there, he can f****** see us”. The prisoner, who is barely conscious, is taken to a mound. The sergeant then asks: “Anyone want to do first aid on this idiot?” One voice says “no”.

Marine C then says: “I’ll put one in the head if you want”. Marine B offers: “Take your pick how you shoot him.” But Marine A cautions: “Not on his head, that’ll be too obvious” and undertakes the shooting himself and warns about the breach of the Geneva Convention. Marine B responds: “Yep, rog: If it ever comes to light, it’ll have been a warning shot.”

The three men then discuss whether to administer first aid to the prisoner, with Marine C saying: “Don’t do it, just pretend to do it”.

The case continues.