The government’s pledge to divert mentally ill people away from the criminal justice system and towards health services is ‘strong on rhetoric’, but understates the extent of the problem, mental health ­professionals have warned.

Justice secretary Kenneth Clarke’s green paper on sentencing and rehabilitation, published this month, quoted a ‘recent survey’ that found 12% of offenders had a mental illness or depression as a long-standing illness, while 20% needed help with an emotional or mental health problem.

Clarke said his ministry would work with the Department of Health to divert ‘less serious offenders’ with mental health issues into treatment rather than prison.

Dr Graham Durcan, associate director of the Centre for Mental Health, said: ‘The paper under-estimates the prevalence of mental ill health among the prison population. The most reliable figures suggest that nine out of ten adult prisoners have at least one mental health problem, and that one in ten has a severe mental illness.

‘Children and young people in the youth justice system are three times as likely as their peers to have diagnosable mental health problems, and many more have emerging mental health problems that are less easy to identify.’

Mental Health Lawyers Association chair Richard Charlton said: ‘The paper is strong on rhetoric, but how much of it can we believe? Diversion [into mental health facilities rather than prison] would stop repeat offending, and protect patients and the public, but the harsh reality is that the money just isn’t there.’