The study found victims saying that their reports to the police were often dismissed or disbelieved.
Many of the 361 people questioned said they were often disbelieved when they sought help after a crime.
The findings come from a three-year study involving academics and the charities Victim Support and Mind.
The report, co-authored by Kings College London, Kingston University and St George’s University of London - in collaboration with University College London - is the first British survey into crime victims which compared those with mental illness against the general population.
The study said that almost half of people with some form of mental illness had experienced a crime in the last year.
It said people with severe mental illness were five times more likely to experience assault, while severely mentally ill women were 10 times more likely to be assaulted.
Six out of 10 women in this group reported being victims of sexual violence as adults, the study said.
But interviewees said that when they sought help, they often found they were treated unfairly by the police and other agencies. Victims said they found it difficult to convince police to take their reports seriously.
In the worst examples, victims said police left them with the impression that they themselves were to blame. Others refused to report crimes out of fear of being detained under mental health legislation.
Victims whose cases went to court reported a range of problems which had an impact on their mental health. One interviewee reported meeting the offender while at court, an incident which triggered an episode of self-harming.
The authors called for police and other agencies to be better trained to help mentally-ill victims of crime and said responses to incidents should be properly monitored and assessed.'Equal right to justice'
Javed Khan, chief executive of Victim Support, said: “It is nothing short of a national scandal that some of the most vulnerable people in our society become victims of crime so often and yet when they seek help they are met with disbelief or even blame.
“It is unacceptable that the criminal justice system fails to meet the needs of people with mental health problems when this report shows all too clearly the terrible impact of crime on them.”
And Paul Farmer, Mind’s chief executive, said: “Being a victim of crime is a horrible experience for anyone to cope with but when you have a mental health problem the impact on your life can be even worse.
“People with mental health problems have an equal right to justice, yet this report reveals that this is not the reality for far too many of us.”
Commander Christine Jones of the Association of Chief Police Officers said it had supported the research because senior officers recognised that the experiences of people with mental illnesses were not widely understood.
“Anyone reporting a crime against them expects to be listened to, taken seriously and treated with respect,” she said.
“Policing and mental health is high on the agenda for chief constables. We support the recommendations in this report, which will further that work.”
The research team interviewed a random sample of 361 people in London who have severe mental illness and the results were compared with official data in the Crime Survey for England and Wales. Further in-depth interviews were conducted with 81 people with mental health problems who had been victims of crime during the last three years.