The former marine sergeant Alexander Blackman, whose murder conviction for shooting dead a Taliban prisoner was quashed this week, should be freed from prison immediately, his legal team are to argue.
A re-sentencing hearing for Blackman, who was jailed in 2013 for killing an injured Taliban fighter in Afghanistan, is to take place on 24 March.
His barrister, Jonathan Goldberg QC, will plead in mitigation and is expected to argue that he has already served long enough in jail and should be released.
Blackman has served almost three and a half years in civilian prisons, the equivalent of a seven-year term in terms of parole eligibility. He will appear at the re-sentencing via video link from Erlestoke prison in Wiltshire.
Blackman was due to remain behind bars until 2021 before he could be considered for parole, but on Wednesday the court martial appeal court ruled that the murder conviction should be quashed and replaced with manslaughter on the grounds of diminished responsibility.
The judges heard that he had been suffering from a mental health condition called adjustment disorder that impaired his judgment.
Goldberg said Blackman was “grateful and relieved”. He said serving a life sentence, which means a prisoner does not know when he will be freed, had been difficult. “He is hoping the time served will suffice and he will be allowed immediate release.”
Of the former marine’s wife, Claire Blackman, who has led the campaign to have him released, he said: “She’s very emotional, very overjoyed, very grateful for the support she has had from so many members of the public.”
A source close to the campaign said Blackman had had job offers and was also considering writing a book about his experiences.
There are concerns, however, that he could become a terrorist target and the same source said the special security measures already in place for his wife would be beefed up when he is freed.
The thriller writer Frederick Forsyth, one of the most vociferous campaigners for Blackman, is calling for an inquiry into the Royal Marines’ handling of him and his comrades in Helmand.
A witness statement written by the former marine colonel Oliver Lee and seen by the Guardian claimed the leadership and oversight of Blackman by his commanders in Helmand in 2011 was “shockingly bad”.
Forsyth said: “I think the Marine Corps need to have an inquiry into what went wrong. The reaction of the senior ranks of the Marine Corps as to turn their backs on a loyal servant. They may have to be held to account for that.
“If the judiciary have any integrity they will order an inquiry into the court martial [at which Blackman was found guilty of murder]. It was a thoroughly unhappy episode in British judicial history. The court martial merits forensic examination.”
Blackman has been doing an Open University degree in prison. He also helps to organise sports events, counsels fellow inmates and teaches maths. When there was a disturbance at the prison he helped to quell it.
Neil Greenberg, a professor in defence mental health at King’s College London and one of the psychiatrists whose diagnosis led to the quashing of the murder conviction, said there should be automatic mental assessments for service personnel accused of serious crimes.
“It’s strange he wasn’t at least assessed,” he said. “I’m really unclear why his unit didn’t provide him with the standard levels of support that were available. The military have plenty of good procedures. The question is why they weren’t applied.”