By Angus Crawford
A growing number of former soldiers are finding themselves behind bars
Data protection rules are preventing the number of military veterans in prison from being discovered, BBC News has learned.
Figures suggest 8,500 of the UK's prison population of about 93,000 men and women may have been in the forces.
Probation officers have raised concerns that increasing numbers of former service personnel are going to jail.
But the Ministry of Justice said the consent of prisoners involved would be needed to carry out accurate research.
Home Office research in 2001, 2003 and 2004 suggested that between 4% and 6% of the total UK prison population may have once been in the military.
More recent figures based on a small sample by pressure group Veterans in Prison suggested the figure could be as high as 9%.
The National Association of Probation Officers warned that could mean as many as 8,500 people, and described the figure as an "alarming" level.
The anecdotal evidence of former service people who have found themselves behind bars lends credence to fears of a surge of ex-military prisoners.
Danny, who is not being fully named to protect his identity, was a soldier before he was a prisoner.
In 2004 he was sent to Iraq, where he had to search would-be suicide bombers.
He said: "I've seen the mindless violence meted out to children out there, it affected me quite deeply."
The MoD hopes to start a new questionnaire for new prisoners
When he returned from Iraq his behaviour became increasingly erratic.
"I started drinking, taking drugs. I would go out two or three times a day to patrol around my flat," he said.
Danny then attacked a drug dealer with a gun, and was sent to prison for three years.
He was later diagnosed with post-traumatic stress disorder and had to wait "a very, very long time" to get specialist treatment, which only came after the Royal British Legion wrote to the prison governor.
In jail he said he met many former soldiers.
"A startling number, the landing I was on had 120 prisoners, there were eight, nine maybe 10 of us," he said.
Nobody knows how exactly many people like Danny are behind bars across the UK.
Such a gap in the understanding of the make-up of the prison population has been condemned by Elfyn Llwyd, the Plaid Cymru MP for Meirionydd Nant Conwy.
He said: "I'm disappointed the government hasn't been more proactive on this. There is no excuse for not knowing the accurate figures."
The Ministry of Defence said it was attempting to obtain better research, that it hopes to conduct a snapshot survey and start a new electronic questionnaire for new prisoners.
But it said there are problems with data protection laws.
The Ministry of Justice, which has responsibility for prisons, said it was working with the MoD on this issue, but a spokesman said "part of that process involves obtaining consent from the individuals concerned".
Fitting back into civilian life can prove challenging for some ex-military
Obtaining consent could involve thousands of prisoners, but Professor Simon Wessely says concerns about data protection are irrelevant.
The director of the Kings Centre for Military Health Research in London said: "No one is asking for confidential medical data."
Professor Wessely also said that finding out the raw number of ex-military prisoners was not as important as finding out why they are in prison in the first place.
He said: "Is it… disadvantaged backgrounds, traumatic experiences or more related to alcohol?"
Danny believes there are a number of reasons behind the trend.
In his own case he said military training conditioned him to respond in certain ways, and when he was threatened by a drug dealer he instantly responded with "lethal force".
However, he conceded "not all soldiers are angels and a certain percentage would have ended up in prison no matter what their circumstances".
He added: "I'm not a martyr, I'm not a victim. I can't point the finger at the military."