The video showed Blackman shoot the insurgent in the chest, telling him to “shuffle off this mortal coil, you c***.” He then told the rest of his patrol to keep quiet because he had broken the Geneva Convention.

Judge Blackett dismissed Blackman’s defence that he thought the insurgent was already dead when he fired the shot, saying it lacked “any credibility and was clearly made up after you had been charged with murder.”

He said the stricken Taliban fighter, who was badly wounded after being hit by cannon fire from an Apache helicopter gunship, was “entitled to be treated with dignity, respect and humanity”.

He said: “The tone and calmness of your voice as you commented after you shot him were matter of fact and in that respect they were chilling.”

Judge Blackett said: “This was not an action taken in the heat of battle or immediately after you had been engaged in a firefight.”

Blackman was under no immediate threat and was in complete control of himself during the murder.

The judge said: “You treated that Afghan man with contempt and murdered him in cold blood.”

“By doing so you have betrayed your Corps and all British service personnel who have served in Afghanistan, and you have tarnished their reputation.”

The case is unprecedented in modern times, with no serviceman having been convicted of murder on a foreign battlefield since at least the Second World War.

Lawyers for Blackman said he was devastated by the sentence and would appeal.

Before sentence, the military court in Bulford heard Blackman was described as a “quality act” by superiors and was in line for promotion to company sergeant major and beyond before the killing ended his career.

Writing in mitigation, Lt Col Simon Chapman, his commanding officer, told the court Blackman was “a caring and devoted family man” who had a bright future with the marines.

However he said “regrettably none of this talent and ability will ever be realised by Sgt Blackman as a Royal Marine.”

He said: “His momentary and fatal lapse of judgement on the battlefield two years ago not only served to end an enemy combatant’s life prematurely, but it has also altered his own life, and that of his family, immeasurably.

“He had so much to behold –a proud career and a promising future. Sadly this is no longer the case. But, fundamentally, he is not a bad man. In fact in almost every respect he is a normal citizen tainted only by the impact of war. However his good work has been undone and his reputation and standing are lost. His career is over – he has no future as a Royal Marine.”

Anthony Berry QC, defending, said the marines had been mentally drained and frustrated during a hellish tour of Helmand province. The marines, based in a small checkpoint in northern Nad-e Ali district, had been used to lure Taliban fighters out. Daily patrols meant the constant risk of bomb attack or ambush.

Mr Berry said: “Parts of their comrades had been hung in trees in order to goad their colleagues and there was the knowledge that if one of them was captured by the Taliban, they would meet a horrible end, possibly being skinned alive and beheaded.

A consultant psychiatrist report concluded Blackman had suffered fatigue, poor sleep, grief from the recent death of his father and “the feeling, though unspoken, of paranoia that he was there to be shot at every time he went out.”

The marines had come to feel “hatred” for the Taliban, who would pose as harmless farmers, while plotting to kill as many British troops as possible.