The Mental Health Act, which was introduced in 1983, is intended to help doctors deal with patients who may have a psychiatric disorder.
Under the Act, patients can be sectioned or detained against their will and given treatment.
BBC News Online examines these powers and what they mean for patients.
Who can be 'sectioned' under the Mental Health Act 1983?
People with different types of defined "mental disorder" can be admitted to hospital against their will under the Act.
These include those with "severe mental impairment", "psychopathic disorder" or "mental illness".
What happens during the sectioning process?
A patient can be sectioned if they are perceived to be a threat to themselves or other people.
Generally, a patient can only be sectioned if two doctors and a social worker or a close relative of the patient believe it is necessary.
One of these doctors is usually a psychiatrist. The other is often a doctor who knows the patient well.
However, in an emergency one doctor's recommendation may be sufficient.
An approved social worker also has to be involved in the assessment, and has to agree that being sectioned is the best course of action for that patient.
The social worker then makes the application for a place in secure accommodation for the patient.
Sectioning a patient enables doctors to assess the extent of the patient's mental disorder and, if necessary, to administer treatment.
How long can patients be detained?
If a patient is sectioned as an emergency case, then they are said to be detained under section 4 of the Mental Health Act.
This enables doctors to detain them for up to 72 hours.
If doctors believe that further assessment or treatment is necessary then the patient can be detained under section 2 of the Act.
This means that they can be admitted to hospital and detained for up to 28 days to undergo a full psychiatric assessment.
At the end of the 28-day period, if the medical recommendation is for the patient's stay in hospital to be extended, a further six months can be given under section 3 of the Act.
A patient can be discharged from hospital at any time if doctors believe they are no longer a threat to themselves or anyone else.
What rights does the patient have during this time?
Some treatments can be given to people detained under various sections of the Act, even without their consent.
This is possible if the treatment is believed absolutely necessary to prevent the patient's condition deteriorating while they are in hospital.
Some more powerful treatments and operations, however, require an additional court order under the Act.
These include any operations which destroy brain tissue or aim to stop parts of the brain functioning - such as a lobotomy - and the surgical implantation of sex hormones in order to reduce the male sex drive.
Can a patient or his or her relative appeal against detention?
Every patient should have a named "responsible medical officer" who has a duty to keep an eye on progress.
In addition, the patient themselves can appeal for release to the NHS trust which is detaining them.
The "nearest relative" also has a right to "order the discharge" of a patient.
However, doctors can block this by producing evidence that the patient still represents a risk to the public or him or herself.
If this happens, the relative cannot try again for another six months.
Finally, there is a right of appeal to the Mental Health Review Tribunal, which can order discharge after a formal tribunal hearing.
However, there have been frequent complaints that this process is extremely slow-moving.
How often are these powers used?
Between 1989 and 1990, 16,300 were given compulsory treatment against their will.
In 2000/2001, 26,707 were formally admitted against their will, in addition to 19,570 who went voluntarily to hospital in the first instance but who were then sectioned.