A Royal Marine jailed for murdering a wounded Taliban fighter received a boost to his bid to have his conviction overturned after prosecutors admitted for the first time that he was mentally ill at the time of the killing.

Alexander Blackman, who was jailed in 2013, is attempting to have his murder conviction quashed, after three eminent psychiatrists agreed he was suffering from a condition known as an “adjustment disorder” when he shot the mortally wounded insurgent in the chest.

The Court Martial Appeal Court heard he had been failed by “shockingly bad” leadership, and felt he had been abandoned by senior officers in a “ghastly” command post in Afghanistan’s Helmand province, where marines feared that if they were captured they would be scalped, skinned alive, castrated and crucified.