Mental health campaigners have blamed flaws in the system for the fact that a schizophrenic man was able to walk out of a psychiatric hospital and kill.
Most mentally ill people are not violent
John Barrett stabbed a cyclist to death in Richmond Park, south-west London, less than 24 hours after walking out of Springfield Hospital in Tooting.
Marjorie Wallace, of mental health charity SANE, highlighted a "shocking failure of mental health services".
The service was so hard pressed it could not protect the public, she said.
Ms Wallace said the service lacked both sufficient beds and staff.
People working in the service were sometimes so busy that they simply did not have enough time to give proper consideration to whether an informal patient - such as Mr Barrett - ought to be detained.
The case also reflected the fact that individuals who may be seriously disturbed and at risk were not taken seriously, or warnings of their families and friends heeded, she said.
Powers to detain informal patients
A doctor in charge of an informal patient's treatment can detain him for up to 72 hours by reporting to a hospital manager that an application for a compulsory admission ought to be made
A nurse of the appropriate class can detain such a patient for up to six hours, or until a doctor with authority to detain him arrives
Mental Health Act 1983
She said: "SANE believes that many of the 40 homicides a year committed by people in contact with mental health services, if not predictable, could have been prevented had the patients been properly treated and cared for and not allowed to discharge themselves, abscond or simply leave psychiatric wards, with little effort being made to follow them up or give adequate information to families, police or those entrusted with their care.
"There seems to be a worrying increase in the numbers of patients who are allowed to disappear in the community while they are extremely disturbed, who then commit suicide and occasionally attack others.
"These cases further shatter the public's confidence in the care in the community policy and increase the stigma for the majority of people with mental illness who are never violent."
The reality is, say campaigners, that most mentally ill patients pose no risk to others.
Some critics have suggested the modern approach of treating mental illness in the community may have increased the risk of violent incidents.
But experts say there is little statistical evidence to back this up.
They say violent attacks by strangers are relatively rare, and it would be wrong to deprive many people of their liberty when they pose little threat.
There have been a number of high profile cases of mentally ill people turning into killers.
These include Christopher Clunis, a schizophrenic who killed Jonathan Zito on the platform of Finsbury Park tube station in north London in 1992.
Clunis had stopped taking his medication.
However, cases of this ilk are not common.
Andy Bell, director of communications at the Sainsbury Centre for Mental Health told the BBC News website that around 30-50 murders a year out of a total of 800 could be blamed on mental illness.
"That is still 30-50 too many, but it is a very low risk when you compare it to the number of violent incidents involving alcohol," he said.
Mental illness common
Mr Bell said around 500,000 people in the UK were suffering from a severe mental illness such as schizophrenia or manic depression, which required specialist treatment. In total, one in six people has some form of mental illness.
He said the idea that all mentally ill people posed a threat to society was a stereotype which was not founded in reality.
"The policy of care in the community has allowed hundreds of people to live peacefully and normally as valued members of the community," he said.
"We don't want to go back to a Victorian policy of locking up people away from the community on the grounds that one day maybe they might be a risk.
"It is very, very difficult to predict violence, and so such a policy would require locking away hundreds, maybe even thousands of people in order to prevent one or two tragic incidents."
Violence is not a common symptom of mental illness. Most schizophrenic individuals, for instance, are not violent; more typically, they prefer to withdraw and be left alone.
Mr Bell said statistics showed that a mentally ill person was twice as likely to be murdered as anybody else.
He said support services continued to be under-funded. Mental health as a whole received a third of the increase in resources granted to the rest of the NHS this year, he said.
The draft Mental Health Bill currently going through parliament would enable the forced detention of mentally ill people deemed to pose a risk to the public who refused to take their medication.