A review has found that Royal Marine Alexander Blackman - who is in jail for the murder of a Taliban insurgent in Afghanistan - let professional standards "slip to an unacceptable low level".
The redacted internal report, released by the Royal Navy, said Sgt Blackman showed 'poor leadership' and 'moral disengagement' when he shot a wounded insurgent in 2011.
He is serving an eight-year jail sentence after being convicted of murder by a court martial.
His wife, Claire, is leading a campaign to free her husband, which has gained the backing of MP Richard Drax and author Frederick Forsythe.
Sgt Blackman allowed professional standards to slip to an unacceptable low level at CP Omar.
His poor leadership was a significant contributory factor in the way the insurgent was treated by other members of the patrol.
Sgt Blackman's rank was a significant contributory factor in preventing others within the patrol from questioning his orders or challenging his actions.
Group conformity and the patrols positive past experiences of Sgt Blackman may also have contributed.
Moral disengagement on the part of Sgt Blackman and the members of his multiple was a significant contributory factor in the handling and shooting of the insurgent.
The difficulty experienced by Sgt Blackman in changing from a mind-set which required him to kill an enemy to one which accepted having to administer first aid to an enemy in order to try and save his life, was a contributory factor in his treatment of the insurgent.
The wife of a former Royal Marine from Taunton jailed for life for murdering a Taliban fighter is making a fresh bid to clear his name
Hundreds of people including many veterans have attended a rally in support of jailed former Royal Marine Alexander Blackman
People gather in Parliament Square in London to support a former Plymouth Royal Marine jailed for murder