The Napo report was compiled from more than 70 case studies. Whatever the true figure, it is apparent that soldiers comprise by far the largest occupational group in the prison system. 'It is clearly worrying that a significant proportion of people in the penal system are ex-servicemen and it doesn't say much for the support given to those leaving the military,' said Andrew Neilson of the Howard League for Penal Reform.

'An inability to cope with civilian life, particularly for those who joined the services on leaving school, can certainly lead to offending and see someone swapping one institution for another.'

Often it is those closest to the soldiers who are victims of their violence. The report cites the example of one serviceman who struggled to adapt to civilian life after six years in the army. His relationship with his partner broke down and she stopped him seeing his children because of his heavy drinking. Verbal abuse turned to physical abuse, which led to a jail sentence.

Another soldier ended up in a prison in Humberside for actual bodily harm. According to his probation report, he started drinking heavily after he returned from having served in Bosnia at the age of 19. The soldier said he had not been prepared for what he saw while on peacekeeping duties. For years he could not get the image of people nailed to trees out of his mind.

'The number of soldiers in prison is definitely on the rise,' said Tracey Johnson of Veterans in Prison, which believes there is a link between the intensity of the army's current missions in Iraq and Afghanistan and the number of soldiers currently in jail. 'They're fighting in back-to-back conflicts, coming out and going back again; they haven't got time to recover. There are not enough of them. They don't have the right cover or equipment and they're absolutely knackered.'

The organisation has been inundated with letters from soldiers in prison. In virtually every case it believes that the writers were suffering from post-traumatic stress disorder (PTSD). One father said that before his son was jailed for threatening to shoot another soldier, he had been wetting his bed and in floods of tears because 'he couldn't get Iraq out of his head'.

'He told me they often had to raid buildings where they believed terrorists were hiding,' the man wrote. 'Because he is a big strong lad, he had the heavy machine gun and so had to enter these buildings first and in his words "was shit scared". I told him anybody would be in that situation, but I got the impression he felt it was a sign of weakness.'

David Bradley, 43, developed post-traumatic stress after serving in Northern Ireland. In 2006, he shot his uncle, aunt and two cousins at close range with a pistol he had smuggled into the UK after serving in Bosnia. Several hours later, armed with a nail bomb, a sawn-off shotgun and a pistol with silencer and ammunition, Bradley walked into his local police station in Newcastle and calmly said: 'I have killed four members of my family.'

As the incidence of post-traumatic stress becomes more prevalent there are suspicions that some soldiers will cite combat fatigue as an excuse for their criminal behaviour. 'There are those who say they have it as some sort of amelioration for their actions,' conceded Peter Poole, director of welfare services at the charity Combat Stress.

The Napo report provides some of the most credible evidence to date that stress is a major factor behind the rise in the number of soldiers going to jail. Dozens of clinical psychiatric assessments speak of soldiers suffering from post-traumatic stress when they attack others. Often the disorder is not identified until the soldier enters the prison system.

'Military operations in recent years have placed the armed forces under increased pressures,' said Derek Howard-Budd, head of welfare at the Soldiers, Sailors, Airmen and Families Association. 'Associated issues like PTSD can take a long time before symptoms develop and much longer to be diagnosed.'

Post-traumatic stress has been dubbed 'the hidden wound' - the injury that is never talked about because of the stigma attached to soldiers suffering psychological problems. 'The idea that it is not a "real" condition is inherited from the First World War, where shell-shock among troops was thought to be a sign of weakness,' said Bridget O'Connell, of the mental health charity Mind. 'Now, with a better understanding of the way trauma affects us, this notion is long-since outdated.'

It was not until he was serving life for murder that Tracey Johnson's husband, Jimmy, who was the victim of a bomb attack while serving in the army in Northern Ireland, became aware he had problems. She fears that many more soldiers will end up going the same way. 'Many of them don't even know they've got it,' she said.

Despite heightened concerns about the prevalence of the condition, there are claims that little is being done to assess soldiers' mental health when they return from war zones. What help is available is usually on an ad hoc basis and often available only when they have been incarcerated. Staff at Everthorpe prison in Brough, East Yorkshire, have become so concerned at the lack of support traumatised soldiers receive upon release that they have taken to issuing them with information packs giving details of mental health charities.

Groups such as Combat Stress can be effective, but have limited resources. 'We can only help those who seek help,' Poole said. 'And there are more people than we are equipped to deal with.'

In a statement to The Observer, the MoD said that counselling was available to service personnel at all times, and pointed out that all troops receive briefings before and after deployment to help them recognise the signs of stress.

'We have launched six pilot schemes of community-based veterans' mental health therapists which will be rolled out across the UK,' the MoD said in a statement. 'Veterans can also receive free mental health assessments from a consultant psychiatrist with a military background. This service is also available to veterans in prison.'

But politicians said it was clear that more needs to be done to identify and treat post-traumatic stress at an early stage. The Labour MP John McDonnell, who is secretary of the Justice Unions All Party Group, said it was time for the government to urgently review systems for supporting serving and retired members of the armed services.

Elfyn Llwyd, a Plaid Cymru MP who has become alarmed at the number of his constituents who have served in the armed forces and are now in prison, said that service personnel and their families were being let down. 'If better treatment was available for these servicemen, hundreds, maybe thousands, would not have offended,' he said.

Veterans In Prison draws comparisons with the United States, where soldiers returning from war zones are put through 'decompression courses' where they are assessed by mental health experts before leaving the base.

'Here they just get them altogether in the barracks and ask them who wants to see a shrink,' Johnson said. 'Nobody's going to put their hand up to that.'

From the Observerʬ†