Tens of thousands of patients may need to be assessed to determine if they should be sectioned under the Mental Health Act, under new guidance issued by the NHS Executive. Managers fear a huge rise in workload in having to comply with the requirement.

The move follows a Court of Appeal judgement in early December that a 48-year-old autistic man had been unlawfully detained in hospital.

The man, who lacked the capacity to consent to his treatment, had not been detained under the 1983 act but had been 'informally' admitted.

The NHS Executive has now written to trusts and health authorities saying they need to review existing patients, as well as new patients, to determine whether they are capable of giving consent.

Many patients with learning disabilities and dementia are currently treated without being detained and are denied the safeguards of the act, such as access to mental health review tribunals.

The letter warns: 'Obviously there are potential resource and workload implications.' But it says 'steps should be taken to review the position of existing inpatients who lack capacity as soon as possible'.

The number of patients who may be affected is hard to estimate, but Chris Vellenoweth, special projects manager of the NHS Confederation, said it could be tens of thousands.

'There are benefits from detention, but the fact remains that this ruling would affect a considerable number of people who are satisfactorily accommodated and cared for,' he added. 'We need to be cautious about uprooting people from their appropriate and settled way of life.'

The main workload for hospitals is likely to be in reassessing existing patients, and in the paperwork in preparing for mental health review tribunals.

Managers will also need to review where patients are living - detained patients can only be housed in a hospital or a mental nursing home registered under the act.

Gregory O'Brien, a spokesman for the Royal College of Psychiatrists, said as many as 400 patients in his trust - Northgate and Prudhoe trust in Northumberland - would need to be reassessed.

'We are expecting that most of them would have to be detained under the act,' he said.

'I am acutely aware of the extra workload it will create for myself, my nursing colleagues and management colleagues.'

Tony Goss, director of elderly services at Lewisham and Guy's Mental Health trust, said he expected to have to review most patients in continuing care beds for the elderly. Some continuing care accommodation, provided through a local housing society, may need to be registered under the act.

The trust involved in the Court of Appeal case - Bournewood Community and Mental Health trust in Surrey - was given leave to appeal to the House of Lords but has not said whether it will do so.