The number of former military personnel in prison is almost three times higher than official Government figures, a criminal justice campaign group said.
No Offence said multiple research studies examined recently showed military veterans comprised at least 10% of the overall prison population in England and Wales. The Ministry of Defence denied, the claim and said the figure was 3.4%.
Chief executive Sue Clifford said there were an estimated 3,000 charities and groups providing support for former troops but a better joined-up approach was needed.
She also said that with 20,000 troops returning to civilian life because of defence cuts, the number of ex-servicemen behind bars could grow. "More needs to happen - it is the tip of iceberg," she said.
"We estimate that 10% of prisoners are military veterans, from samples of the prison population. We believe the cuts will have a significant effect on prison numbers as thousands of troops return to civvy street before they were anticipating - so they have not had a chance to plan.
"The current prison system shows that only 3.4% of prisoners are military veterans, which under acknowledges the figure. The data captured when people go into prison is not accurate, as people do not talk about their former role unless asked. Some veterans, even when asked, do not say as there is huge pride associated with being in the military."
Trevor Philpott, a Royal Marines officers for 34 years and founder of the Veterans Change Partnership, told the Independent on Sunday: "There is a sense that the Government is reluctant to address the true figures: if there was the slightest admission of combat causing mental health problems, there is a fear of legal action."
A Ministry of Defence (MoD) spokesman said: "Official Government statistics show the number of ex-service personnel in prison and on probation in England and Wales is 3.4%. An independent report by The Howard League for Penal Reform last year concluded that ex-service personnel are less likely to be in prison than civilians.
"The vast majority of personnel leaving the Armed Forces make a successful transition to civilian life and there is a wide range of help and support for all those that need it, including NHS mental health services tailored to meet the needs of veterans and the MoD's Veterans Welfare Service."
A Ministry of Justice spokeswoman said: "3.5% of the total prison population are veterans compared to 9.1% of the UK population. Though their resettlement needs are broadly similar to other offenders, we work closely with independent organisations that provide services specifically for them."