An independent agency is needed to monitor the mental health of prisoners, a charity has said.

The warning comes as figures show that three quarters of male inmates in England and Wales suffer from two or more mental health conditions, including schizophrenia.

It has also been found that 40 per cent show symptoms of at least one neurotic disorder such as depression or phobias, with one in five prescribed medication such as anti-depressants.

A fifth of male prisoners have attempted suicide at some stage in their lives, with the same proportion admitted for psychiatric care, according to the Prison Reform Trust.

"Proper investment in court diversion, mental health and drug treatment in the community and secure health provision for those who need it would lift the burden off untrained prison staff and put a stop to the cruel and unnecessary punishment of jailing vulnerable people," said director Juliet Lyons.

At a London conference hosted with mental health charity Mind, the PRT is set to argue that more investment is needed in appropriate care and treatment.

It will also call for a renewed focus to ensure the Prison Service meets NHS standards on mental health issues.

Courts should try and use alternatives to custodial sentences for minor offenders with mental health problems, the group will say.

However, the Department of Health has defended its record.

"The government is investing in schemes to divert mentally ill people away from custody, and to provide better mental health services in prisons," said a spokesman.

"The transfer of commissioning responsibility for prison healthcare to the NHS will ensure that prisoners can access the same range and quality of health services as the general public.

"A total of £10 million was spent on NHS mental health in-reach services for prisons in England during the last financial year and this is expected to double to £20 million a year between March 2004 and March 2006. 

"In addition, more than 300 additional NHS staff have become involved in providing mental health services in prisons."