Government proposals to lock up people with 'dangerous severe personality disorder' have come under renewed fire as consultation draws to a close this month.

Measures for the indefinite detention of people judged 'untreatable' under current mental health law are 'completely unrealistic' in their aims, 'based on no reliable evidence' and have 'major' civil liberties implications, say mental health experts.

In a formal response to joint proposals from the Home Office and the Department of Health, the Royal College of Psychiatrists pours scorn on the 'tenuous' link between severity of personality disorder and dangerousness, seen as central to the proposals.

'There is an assumption. . . that the risk they seek to guard against is directly related to personality disorder and that detaining those with the severest personality disorder will, therefore, reduce the risk. There is no reliable evidence that this is so.'

Legislation based on that link 'would make most psychiatrists questioned more cautious about diagnosing personality disorder', the response warns.

The government proposals outline two options. Under a new legal framework, people with dangerous severe personality disorder would be held in new facilities outside the prison and health services, depending on whether they had committed a crime. An alternative option would 'strengthen' legislation by holding those deemed 'dangerous' in prison or hospital.

But the Royal College of Psychiatrists describes detention on the basis of past offences as 'ethically unacceptable, practically difficult, and unnecessary to the aims of these proposals'.

The British Psychological Society's response similarly flags up 'the risk of a high rate of people being falsely diagnosed' using current assessment tools.

The government proposals suggest that about 2,500 people would be affected by the measures, of whom 1,400 are in prison and 400 in secure hospitals. But the BPS believes that the claim that new measures would only affect about 700 people in the community 'seriously underestimates' the numbers likely to be affected.

Meanwhile, mental health charity Mind has stressed the 'very real concerns' of people currently diagnosed as suffering from a personality disorder, 'the vast majority of whom would never pose a danger to anyone, with the possible exception of themselves'.

Parliamentary officer Sue Brown said it was 'vital' that proper research to improve 'levels of accuracy' in diagnosing disorders and risk, and extra resources for services came ahead of legislative change. 'The level of accuracy becomes a much more crucial issue when you are talking about detaining people who have not committed an offence and who cannot necessarily be treated, ' she added.

The National Schizophrenia Fellowship highlighted fears that the government's focus on public safety could increase the stigma attached to mental illness.