Overall 40 per cent of the force studied showed "significant" psychological distress, and reported a much higher level of symptoms of post-traumatic stress disorder than the general population.
Symptoms included flashbacks to incidents, nightmares, inability to sleep or enjoy normal activities, and an over reliance on alcohol and drugs. Women reported almost twice as many symptoms as men.
Officers with the longest service suffered the highest levels of distress, according to Jennifer Mitchell-Gibbs, an inspector in the Essex force, who with Dr Stuart Mitchell, a clinical psychologist at Hartlepool General Hospital, surveyed 1,000constables and sergeants from a force in south- east England.
"This is contrary to expectations. Officers don't become immune to traumatic events over time ... the effect of stress is cumulative; a drip-drip effect and then it may be just one fatal accident too many that is the crunch," she said.
Death, major disasters, and incidents resulting in injury to themselves or colleagues, were the most common problems. However, attendance at a sudden death, a very common policing duty, was described as "very distressing", particularly for those contacting the relatives of the dead person. Some 23 per cent of officers displayed severe psychological distress.
The survey found a heavy emphasis on suppressing emotions, but this prevented officers from asking for help. "It is part of the police culture to project strength, authority, resilience - the `John Wayne syndrome'," Ms Mitchell-Gibbs told the first European Conference on Traumatic Stress in Emergency Services, Peace Keeping Operations and Humanitarian Aid Organisations in Sheffield yesterday.
The British force has adopted United States practices of relying on critical- incident debriefing and peer-group counselling for officers in need of help. But officers in the survey said they wanted external help because they mistrusted their colleagues.
Ms Mitchell-Gibbs said previous studies suggested that the British force was less vulnerable to stress generated by their daily duties, but found bureaucracy, their workload and bad relationships with colleagues more stressful.
The study confounds this view, Ms Mitchell-Gibbs said, and exposes the toll general policing takes on a force.
Another study presented at the conference which challenges accepted views, was that more than three-quarters of people with post-traumatic stress disorder have no previous history of psychiatric problems. However a significant number had suffered a major life experience - both good and bad - in the 12 months prior to the trauma which triggered PTSD.
The study of mental health patients in Nottingham revealed that the most common traumas included road accidents and assaults, and bad birth experiences for women.