More than 2,500 former members of the armed forces entered the prison system last year, with experts warning a disproportionate number were being jailed for serious violence and sexual offences.

According to the Ministry of Justice, veterans represent between 4% and 5% of the UK prison population, raising concerns about the impact of the Afghanistan and Iraq campaigns on mental health issues in the armed forces.

The historic murder conviction against Alexander Blackman, a British marine who shot dead a seriously wounded Taliban prisoner in Afghanistan, was quashed this week and replaced with one of manslaughter on the grounds of diminished responsibility. Blackman’s lawyers argued that he had adjustment disorder at the time of the killing after serving for months on the frontline in terrible conditions.

The MoJ began identifying veterans as they entered the prison service in January 2015 after concerns about the management of ex-service personnel were raised in a review of the criminal justice system.

The figures show that former members of the armed forces accounted for 721 of the “first receptions” from July to September 2015, the first period for which figures were released.

The numbers appear to have fallen since, with 545 arriving in the system in the same period a year later. In the year leading up to last September, 2,565 veterans were jailed.

When the data collection was first announced in December 2014, the then justice secretary Chris Grayling said it would help “identify veterans at the earliest opportunity, so that we can take a more tailored approach to help them turn away from crime”.

Frances Crook, the chief executive of the Howard League, said that several factors contributed to the number of veterans entering the prison system, including alcohol abuse and post-traumatic stress disorder. Research by the Howard League found that 25% of ex-service personnel were in prison for sexual offences, compared with 11% of the civilian prison population.

Crook said: “Members of the armed forces represent about 5% of the prison population, but they represent a disproportionate number of serious violent offences and sexual offences, and that raises questions that need answering. These are not victimless crimes. They have a terrible effect of the victim.”

Sue Freeth, the chief executive of the charity Combat Stress which supports veterans with mental health issues, said that the Ministry of Defence had done more in recent years to help service personnel. “Things are improving – partly because there is less stigma, and partly because there are simply a lot of people affected so people know more about it. People are coming for help earlier, too, which is important.”

She said it was critical that families were supported, as well as those operating in dangerous situations. “We see children who are effectively part-time carers. It affects everyone.”

Richard Streatfeild, who served in Afghanistan in 2009 and wrote Honourable Warriors: Fighting the Taliban in Afghanistan, said that problems often emerged after soldiers had left the army.

Streatfeild said: “You see people start to drink too much, and then there are discipline issues, and then the relationship goes, and then suddenly they’re really struggling.

“When they’re still in the army, they are easy to identify, and everyone knows what is going on. But it is when they transfer to civilian life that it gets very complicated because people don’t realise what they have been through.”

During six months in Helmand province, Streatfeild and his men engaged in more than 800 firefights and were the target of more than 200 improvised explosive devices. Ten men in his company were killed and 50 were wounded.

Prof Sir Simon Wessely, the president of the Royal College of Psychiatrists and co-director of the King’s Centre for Military Health Research, said it was important to acknowledge all the factors affecting soldiers’ mental health.

He said: “We know that most service personnel don’t come back with mental health problems, though nearly all of them come back as different people. They are changed by their experiences, but that is not a mental health problem.

“It’s never just about what happens on the battlefield, it’s about an interaction between the people we recruit, what happens to them, and the societies that come back to. It’s always a combination of all three.”

Patrick Rea, a director of PTSD Resolution, said that the charity saw criminality and substance abuse among ex-service personnel.

“Most veterans are very disciplined, so their behaviour tends to be very self-harming,” he said. “They quite often find us because their partner has told them: ‘You have to get help because I can’t do anything more’.

“But they do need to want help, too. A lot of veterans don’t believe they can get better, so they live in a state of distress. They soldier on. I would just like to tell them that they can get better. There is a way.”

A spokesperson for the Ministry of Defence said: “Most former service personnel return to civilian life without problems and are less likely to commit criminal offences than their civilian counterparts, but we’re determined to help those who fall into difficulty, and last year awarded £4.6m to schemes targeted at tackling this issue.

“The government has enshrined the Armed Forces Covenant in law to make sure veterans are treated fairly and receive the support they deserve, including with mental health issues, getting on the housing ladder, and applying for civilian jobs.”

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