A Royal Marine imprisoned for murdering a wounded Taliban fighter could be freed in months or even weeks his supporters hope, after senior judges quashed his conviction.

Alexander Blackman’s crime should be downgraded to manslaughter because he was mentally ill at the time of the killing and under “quite exceptional” stress, an appeal court ruled.

The 42-year-old former sergeant now faces a substantial cut in sentence at a new hearing expected later this month and could even be released immediately because of time already served.