The Home Secretary will give details next Monday of the scheme under which up to 2,000 people with dangerous personality disorders could be moved into new secure units.
The idea has already run into criticism from civil liberties groups who say the intention to lock up some individuals even if they have committed no crime is a "bridge too far".
The plans will also be extremely costly to implement. Harry Fletcher, of the National Association of Probation Officers, said 10 units would be needed. "They would be far more expensive than prisons, which cost pounds 25,000 a year per place. These units would need specialist staff and would cost in the order of pounds 100,000 a year for each individual."
Mr Straw's plans, announced in February, will be outlined in greater detail at a Home Office conference for psychiatrists and other specialists in the field. If legislation is passed, England and Wales will be the first countries in the world to lock up so-called untreatable people before they have committed crimes.
Under the plan, a Medical Legal Panel would be given powers forcibly to detain people without limit of time.
Current prisoners deemed to pose a risk and known psychopaths in the community would be assessed by a team of probation, health, prison and social services staff. Those detained would have their case regularly reviewed and could appeal against their detention.
The move is designed to close a legal loophole which states that people can only be detained if doctors believe they will respond to treatment. Around 2,000 people in England and Wales are thought to suffer from severe personality disorders. The Government ordered the consultation and reform of the Mental Health Act after Michael Stone was convicted last year of murdering Lin Russell and her daughter Megan, and attempting to murder Dr Russell's other daughter, Josie, in a hammer attack. Stone had been repeatedly released from mental health units because doctors said his personality disorder was untreatable.
Paul Cavadino, of the National Association for the Care and Resettlement of Offenders said: "Locking people up who have not offended would be a bridge too far ... There is a real risk of detaining indefinitely people who would not have gone on to commit serious crimes."
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