Psychologists discovered that just over one in five servicemen on deployment showed signs of psychological distress and less than four per cent showed signs of post-traumatic stress disorder (PTSD).
These levels of mental health are the same as soldiers, sailors and airmen in training and less than those in other high-stress occupations, such as police officers, doctors in emergency departments and disaster workers.
Focusing on the mental health of UK armed forces while on deployment, the study by researchers at the Institute of Psychiatry, King's College London, included servicemen and women at eight locations across Iraq in January and February 2009.
Of the 611 surveyed, 20.5 per cent demonstrated symptoms of psychological distress, while 3.4% showed signs of PTSD.
Respondents were more likely to report good health if they were of officer rank, if they felt their unit was very cohesive and had supportive leadership, and if they had taken a period of rest and recuperation in an area outside an operational theatre.
Those who reported psychological distress were most likely to be young, female, in the Army, and of junior rank.
Personnel belonging to junior ranks were also more likely to show symptoms of PTSD, along with those who felt they were in danger of being killed.
The report showed that sickness levels were very low with only 1.6 per cent of personnel taking more than three days off during their six month deployment.
While a lot of research on the mental health of UK armed forces personnel has been conducted either before or after deployment, very little is known about their mental health during employment, Professor Neil Greenberg from the Academic Centre for Defence Mental Health said.
"Interestingly, those who told us they remembered having a pre-deployment stress briefing reported significantly better mental health than those who did not," he said.
Although most units had medical support, Prof Neil said training for medical staff had only recently begun to be standardised to ensure it covered mental health disorders.
"Improving training, as well as raising awareness among staff of the link between these personnel reporting sick and having poorer mental health, may help identify those in most need of psychological help," he said.
Those who took part in the survey, published in the British Journal of Psychiatry, represented about 15 per cent of personnel deployed in Iraq at the time.