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Young British men are much more likely to commit violent crimes if they have served in the armed forces, a study has found.

Of around 3,000 military men under the age of 30, more than a fifth, had a conviction for violent offences, compared with 6.7 per cent of their street peers.

There were strong links between combat experience, post-deployment alcohol misuse, traumatic stress and violence.

Men who had seen combat in Iraq and Afghanistan were 53 per cent more likely to commit a violent offence than comrades given non-combat roles.

In the biggest study of its kind ever undertaken, researchers were given access to police records on almost 14,000 randomly selected men and women who were active or former members of the armed forces, mostly the army.

Participants provided information about their experiences before and after joining the military and underwent psychological tests.

A search of the Police National Computer was made for any convictions, cautions or warnings relating to the study population.

Overall, 17 per cent of the men had criminal records, and 11 per cent had committed violent offences.

The findings, released on the 10th anniversary of the Iraq War, are published in a special issue of The Lancet medical journal.

Some people with aggressive dispositions make very good soldiers, that’s the nature of the game

Study leader Deirdre MacManus, from King’s College London, said: “There has been a lot of media coverage and public debate about violence committed by veterans of the conflicts in Iraq and Afghanistan.

“Our study, which used official criminal records, found that violent offending was most common among young men from the lower ranks of the army and was strongly associated with a history of violent offending before joining the military.

“Serving in a combat role and traumatic experiences on deployment also increased the risk of violent behaviour.”

Violent offences covered a broad range of acts, from verbal harassment to homicide. They did not include incidents of domestic violence.

Sir Simon Wessely, co-director of the Centre for Military Health Research at King’s College London, who co-authored the study, admitted that an aggressive streak can be useful in a soldier.

“Some people with aggressive dispositions make very good soldiers, that’s the nature of the game,” he told a press conference in London.