A royal marine murdered an injured insurgent in Afghanistan by shooting him in the chest at close range, a British court martial board has found.

According to prosecutors, he told fellow soldiers to hush it up and acknowledged “I just broke the Geneva Convention.”

The board ruled that the commando, who can only be identified as Marine A, was guilty of killing the unnamed man in Helmand Province in September 2011.

“It is a matter of profound regret in this isolated incident that one marine failed to apply his training and discharge his responsibilities,” Brigadier Bill Dunham, of the Royal Marines, said in a statement.

“It was a truly shocking and appalling aberration. It should not have happened and it should never happen again.”

Prosecutors said the incident took place after a military base in Afghanistan’s Helmand Province was attacked by two insurgents. A helicopter opened fire in response, and Marine A, together with two other British soldiers, then discovered the injured Afghan in a field.

The three moved the man to a sheltered area. Marine A shot the Afghan in the chest with a 9mm pistol, before quoting a phrase from Shakespeare to “shuffle off this mortal coil” as the man died before him, according to prosecutors.

He then turned to his comrades – identified only as Marines B and C – and said: “Obviously this doesn’t go anywhere, fellas. I just broke the Geneva Convention.”

Marine B then replied: “Yeah, roger, mate.”

The exchange was recorded in video inadvertently shot by a camera mounted on the helmet of Marine B. The soldiers were arrested when police found the video on a soldier’s laptop. Footage was shown at the court martial.

The three denied murdering the Afghan, but a seven-member board convicted Marine A after a two-week trial. The two other marines were accused of “encouraging and assisting” in the killing, but they were acquitted on Friday.

The mandatory sentence for murder as prescribed by law is life imprisonment but the court will decide what minimum sentence should be served.

The convicted soldier will be sentenced on December 6.