Jeremy Coid, of St Bartholomew's and the Royal London School of Medicine, said the public should have no illusions about the "inexact science" of risk assessment of people with mental disorder.
He was speaking at a London conference where Frank Dobson, the health secretary, and Jack Straw, the home secretary, had outlined the proposals to change the law to enable detention of more than 2,000 people deemed to constitute a "very high" risk to the public.
The ministers stressed that the system would be carefully structured to comply with human rights law.
But Professor Coid, who is also director of the forensic psychiatry research unit in Hackney, east London, said no system could guarantee there would be no wrongful detention.
"I do feel it's important that people's expectations are realistic," Prof Coid said. "The public and the media must accept that risk assessment is not an exact science." His comments will be seized on by mental health and civil liberties groups which expressed grave reservations about the government's plans, which are out for consultation until the end of the year.
A coalition of some 20 such groups, including Mind, Liberty and the Law Society, said it was wrong in principle to detain anybody who had not committed an offence. The proposals would create a disturbing precedent in an area where diagnosis was notoriously difficult.
The National Association for the Care and Resettlement of Offenders said it welcomed the idea of separate services for offenders with severe personality disorder, but considered it an "excessive reaction" to extend the idea to non-offenders.
The Royal College of Psychiatrists declared that the proposals raised profound issues of civil liberties. A spokeswoman said the college would be "strongly opposed" to the detention of people against their wishes if they were considered to have an untreatable condition.
Ministers intend to act following growing concern that dangerous people with severe personality disorder are being discharged into the community, or refused admission to hospital, because some psychiatrists think them untreatable.
Of the people expected to be detained under the proposed powers - almost all of them men - some 1,400 are in prison, 400 are in special hospitals and between 300 and 600 are in the community.
Under the proposals, these people would be detained "indeterminately" after a full assessment of their condition. They would have a right of appeal and their cases would be reviewed regularly to see if they had stabilised sufficiently for it to be safe for them to return to the community.
The consultation paper on the plans sets out two possible approaches: amending existing laws to facilitate detention within the prison and hospital services, or legislating to create a separate service specifically for severe personality disorder cases.
Mr Straw said the idea of preventive detention had been enshrined in law for 200 years. But critics pointed out that it had been limited to cases where doctors considered the individual likely to benefit from treatment.
No other country detains non-offenders in the way envisaged by ministers, and the consultation paper does warn that "the absence of treatment might raise issues" under the European Convention on Human Rights.
