A Royal Marine found guilty of murdering an injured Afghan fighter has won a sentence reduction because of the combat stress disorder he was suffering from at the time of the incident.

Although the Court Martial Appeal Court rejected a conviction challenge by Sergeant Alexander Blackman, three leading judges cut his minimum term from ten to eight years - the least time he must now serve before becoming eligible to apply for parole.

Lord Chief Justice Lord Thomas, Sir Brian Leveson and Lady Justice Hallett, giving their ruling in London, said that the particular stresses affecting Blackman should have been "accorded greater weight as a mitigating factor" when he was sentenced by a court martial in Bulford, Wiltshire, following his conviction last November.

Allowing an appeal against sentence, Lord Thomas said: "On all the evidence before us it is clear that in the events surrounding the murder of the insurgent, the appellant acted entirely out of character and was suffering from combat stress disorder."

As well as the imposition of a life sentence Blackman, now 39, was "dismissed with disgrace" from the Royal Marines after he had served with distinction for 15 years, including tours of Iraq, Afghanistan and Northern Ireland.

The killing happened in Helmand province in 2011 while Blackman, was serving with Plymouth-based 42 Commando.

He shot the Afghan, who had been seriously injured in an attack by an Apache helicopter, in the chest at close range with a 9mm pistol before quoting a phrase from Shakespeare as the man convulsed and died in front of him.

Blackman told him: "There you are. Shuffle off this mortal coil. It's nothing you wouldn't do to us."

He then turned to comrades and said: "Obviously this doesn't go anywhere, fellas. I just broke the Geneva Convention."

During the trial Blackman, who denied murder, said he believed the victim was already dead and he was taking out his anger on a corpse.

He has said he felt ashamed at his lack of self-control, describing it as a "stupid lack of self-control and lapse in judgment".

Reducing the minimum term, Lord Thomas said: "It is very unfortunate that the only medical evidence before the court martial and before us was obtained over two years after the murder.

"However, in assessing the evidence of stress and its effect on the appellant, we attach particular importance to the evidence in relation to the remoteness of the command post at which the appellant had been stationed for five and a half months and the limited contact with those commanding him.

"His mental welfare had not been assessed in the way in which it would ordinarily be assessed by a commanding officer."

The judge added: "On that basis we have therefore concluded that although he remains subject to a sentence of imprisonment for life, the minimum term which he must serve before being considered for parole should be reduced to eight years. His release will then depend on the Parole Board."

An MoD spokesman said after the ruling: "We respect the authority and decision of the appeal court, and it would be inappropriate for us to comment on the sentence."