Taliban prisoners in Afghanistan, 2011 (stock photo)

A seven-strong military board found Sgt Blackman guilty of murder last year.

Sentencing him in December, Judge Advocate General Jeff Blackett told him he was a disgrace to the British armed services and that the video footage of the incident showed he was not "under any immediate threat" when he fired his weapon. Other soldiers had faced similar strains without reacting in the same way, he said.

The case has already proven to be deeply divisive.

Gen Sir Nick Houghton, the head of the Armed Forces, has said that "murder is murder" and labelled the incident a "heinous crime".

But more than 45,000 people have signed a Downing Street petition calling for Sgt Blackman's immediate release.

"He’s held his hands up. What he has not done in my eyes is commit murder. He genuinely thought, and I have absolutely no reason to disbelieve him, that that insurgent was already dead," said his wife.

"He should not have discharged his weapon into him – it was the madness of the moment – and he sure as hell wished he hadn’t either but he is not a murderer."

Marine 'A': Criminal Or Casualty Of War? will air on BBC One on Wednesday 9 April at 10:35 BST

Audio recording of the incident in 2011

(warning: may cause distress)

"He bitterly regrets it": murder marine's wife appeals for his release