The conviction of a Royal Marine, known as ‘Marine A’, for the murder of a Taliban fighter may be quashed, as review of the case found that Britain’s most senior military judge mishandled the trial.

The Criminal Cases Review Commission (CCRC), the body which investigates potential miscarriages of justice, has concluded that Jeff Blackett, the Judge Advocate General, was guilty of an “apparent failure” to properly apply the law by failing to give a military jury the option to convict Alexander Blackman of the lesser charge of manslaughter, according to extracts of an official report seen by The Telegraph.

Blackman has served more than three years in prison for shooting dead a mortally wounded insurgent in 2011, but was granted the right to appeal last month after the CCRC was given new psychiatric evidence showing that he was suffering from a then-undiagnosed mental disorder at the time of the incident.

Blackman denied murdering the insurgent at his original court martial, claiming he thought the insurgent was already dead when he shot him in the chest with his pistol at close range, before quoting Shakespeare.

But the CCRC has found that even if the new evidence had not come to light, Blackman would be eligible for a retrial as Judge Blackett failed to offer the board of seven officers the option to convict him of manslaughter if they believed he thought the insurgent was already dead when he shot him.

The report states: “If it was a possibility that the board would find that, factually, Mr Blackman had caused or hastened the victim’s death, then it should have been empowered with the option of returning a verdict of guilty of manslaughter, depending on the view it took as to Mr Blackman’s intention.

“In the event, [Judge Blackett] did not direct the board that manslaughter was a legitimate option, although the CCRC concludes that Mr Blackman’s legal representatives should have anticipated that such a direction could be made.

“The inability of the board to make such a finding potentially amounts to a material irregularity.”

It concluded: “Due to the failure to leave to the board the option of returning a verdict of guilty to unlawful act manslaughter, if the board was satisfied that, factually, Mr Blackman’s shot had hastened the death of the victim, the CCRC considers that there is a real possibility that the Court of Appeal would find Mr Blackman’s conviction to be unsafe.”

Previously disclosed excerpts of the review revealed that Blackman’s defence team had also come in for criticism thanks to a number of “deficiencies in the standard of defence”. One of those deficiencies was the failure to ask Judge Blackett to consider manslaughter as an option. However, the new material shows the CCRC found it is the “ultimate responsibility of the trial judge” to be aware of the possibility.

Blackman’s appeal will be heard in two parts. The first, focussing on psychiatric evidence, is scheduled to commence on February 8 in front of five senior judges.

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