Media playback is unsupported on your device Media caption Helmet-cam video shows marines witnessing the Apache helicopter crew firing at the insurgent

A Royal Marine had a recognised mental illness when he fatally shot an injured Afghan fighter, the Court Martial Appeal Court has heard.

Sergeant Alexander Blackman, 42, from Taunton, Somerset, is appealing against his murder conviction.

Blackman, known as Marine A, was sentenced to life in 2013.

His defence team argues fresh psychiatric evidence would have provided him with the "partial defence of diminished responsibility".

Blackman watched the court proceedings on Tuesday via video link from jail.

Jonathan Goldberg QC, for Blackman, said that at the time of the 2011 shooting the marine was serving with Plymouth-based 42 Commando in Helmand province, Afghanistan, in conditions which were a "breeding ground" for mental health problems.

Mr Goldberg said three experts agreed that at the time of the killing, Blackman, described as a reserved man like a "John Wayne character", had been suffering from an adjustment disorder.

'John Wayne character'

He argued that this affected Blackman's ability to form a rational judgement or exercise self-control and crucially to know whether the insurgent was alive or not.

Blackman shot the insurgent, who had been seriously injured in an attack by an Apache helicopter, in the chest at close range with a 9mm pistol.

Blackman used abusive language as he said: "There you are. Shuffle off this mortal coil."

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

The shooting was captured on a camera mounted on the helmet of another Royal Marine.

Image copyright PA Image caption Sergeant Alexander Blackman was convicted of murder in 2013

Blackman was convicted in November 2013 and sentenced to life with a minimum term of 10 years.

In May 2014, the Court Martial Appeal Court rejected a conviction challenge, but reduced the minimum term to eight years because of the combat stress disorder he had been suffering from.

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

'Breaking point'

Prof Neil Greenberg told the appeal hearing that everybody had their "breaking point".

"There is no such thing as a Rambo type, an Arnold Schwarzenegger soldier, who can face all sorts of stresses and appear to be invulnerable.

"That sort of person only exists in the cinema."

He said that, assuming that the video showed elements of planning and deliberation, it was completely consistent with an adjustment disorder.

But Richard Whittam QC, for the Crown, said that the mere fact of an adjustment disorder did not get one "through the door" of diminished responsibility.

The hearing continues.