Managers and patient groups have welcomed a Law Lords ruling which will mean thousands of people detained under the 1983 Mental Health Act can be treated as voluntary patients again.
But the case involving Bournewood Community and Mental Health trust and an autistic man in his 30s known only as L has sparked new calls to scrap the 1983 legislation.
The Law Lords last week overturned an Appeal Court ruling from December last year that the man, a voluntary patient, was illegally detained as he could not consent to treatment.
After the December ruling the Department of Health ordered all trusts to reassess patients.
An estimated 40,000 patients were then sectioned under the Mental Health Act. Now junior health minister Paul Boateng has promised guidance.
NHS Confederation chief executive Stephen Thornton welcomed Mr Boateng's promised guidance on assessment and admission. He said: 'The priority must be that human rights are protected.'
David Pamment, Bournewood's mental health services director, said: 'This was a case waiting to happen, if not at Bournewood then elsewhere. There are insufficient safeguards on mentally incapacitated patients.'
Had the Appeal Court decision been upheld, the trust faced additional costs of pounds 100,000 a year to detain around 100 patients under the Mental Health Act.
The trust has introduced a system of second opinion, carer involvement, advocacy and other checks to ensure consent. Mr Pamment said such a system could be included within the Mental Health Act Commission's remit.
Calling for a review of the 1983 act, Mary Teasedale of the National Schizophrenia Foundation, said: 'The whole creaking tribunal system would have been stretched to breaking point if the original decision had been upheld.'
A spokesperson for mental health charity Sane called for the introduction of a mental incapacity bill.
This would provide a statutory framework and necessary protection for affected patients, the organisation says.