Former SAS soldier Andy McNab tells a Howard League for Penal Reform inquiry that lack of education is the real reason for the large number of ex-servicemen in prison

One of Britain's best-known soldiers has dismissed post-traumatic stress disorder (PTSD) among the armed forces as little more than an excuse for recruits to leave the service early.

Andy McNab, the former SAS soldier now turned best-selling author, said servicemen and women "were very resilient" and that the perception that significant numbers of them suffered PTSD was wrong.

McNab, who spent the best part of two decades in the army, said there was a trend for armed forces personnel wanting to leave before the end of their contract to claim they had PTSD in order to obtain a medical discharge and a pension.

"It is starting to be perceived as an honourable excuse for leaving prematurely," he said. "It is in effect the new 'back problem', an injury that is very difficult to diagnose."

McNab made the comments before an inquiry into the number of former armed services personnel who are in prison. The inquiry has been established by the Howard League for Penal Reform in response to concerns that a disproportionately large number of soldiers are ending up behind bars. The probation service, Napo, estimates there are some 8,500 former service personnel currently in prison – around 10% of the total prison population. The Ministry of Justice puts the figure at 2,500.

The inquiry's findings are likely to be studied closely by both the government and the armed forces. Several former service chiefs – including Admiral the Lord Boyce, the former head of the armed forces, and General Guthrie, former head of the army – have attended its evidence sessions. A report based on the inquiry's findings is due to be published in November to coincide with Armistice Day.

McNab identified a lack of education as a key factor in why such a high number of servicemen and women were ending up in jail. "An average reading age within infantry is 11, but the army will recruit from a reading age of seven," McNab told the inquiry. "The usual period an infantry soldier will stay in the army is between four to six years. When they leave the army, they will usually go back to the same environment they came from and often tried to escape by joining the army. They still have low literacy levels and, as infantry is very transient, no attachment to any other place. It is vital to get numeracy and literacy up, so that there is a broadening of horizon for these service leavers."

He said that, with few qualifications or skills, a person's employment chances were limited once out of the army, where its "tribal system" had looked after them.

Given their lack of preparation for civilian life, it was inevitable, he said, that some would encounter problems and end up in the criminal justice system. "The Ministry of Defence should recognise that it has a responsibility to progress the recruits, to educate, raise their aspirations and change their horizons for when they leave the army," McNab said. "The MoD need to understand that from day one it should be drummed into recruits that one day they will leave the army, and you need to get prepared for that day. That is not happening at the moment."

Andrew Neilson, assistant director of the Howard League, said that McNab's comments helped to shine new light on the reasons why former service personnel end up in prison.

"Our inquiry is hovering around this question of the extent to which former soldiers are in the prison system because of the pressures of active service, and the extent to which the real culprit is actually poverty," Neilson said. "The army traditionally finds recruits from disadvantaged communities and many will return to those communities on leaving the services."

Neilson added: "The argument that the army could do more to enable social mobility and raise horizons is very much worth exploring, as that should have a direct impact on the numbers of veterans ending up in the criminal justice system. At the same time, however, there is a lot that mainstream services such as health and housing should also be doing to ensure we do not fail those who have served our country."

McNab's comments about PTSD are likely to provoke controversy. In the US, research published by the National Institute of Mental Health estimated that 19% of Vietnam veterans experienced PTSD at some point after the war.

But McNab told the inquiry: "The reason our armed services are more resilient to PTSD than, say, the US armed forces is that in the US there is a heavier reliance on the use of reserves. There is a distinction between the military and the reserve, people coming into the system for a year who have not got the depth and, although they are present physically, emotionally they are not part of the tribe."