People have been put at risk because the NHS has stopped funding the automatic investigation of all killings by mental health patients, according to psychiatrists and victims’ families.

Experts who had looked into every such homicide for 20 years had to stop doing so last year after NHS England stopped paying the £100,000-a-year cost involved, the Guardian has learned.

An estimated 120 people a year across the UK are killed by someone who is mentally ill. Such killings account for 11% of all homicides. About a quarter of victims are children; many others are relatives or friends of the assailant and some are strangers killed in random attacks.

Previously, for 26 years until last year, researchers from Manchester University had looked into the mental health history and NHS care received by the perpetrator of every such homicide to try to identify patterns and flaws which could be tackled to reduce the risk of similar attacks in the future. Their findings had led to improved care of potentially dangerous mental health patients.

“This is a risky and reckless decision. A lot of families will feel that their suffering doesn’t matter because of this decision, because no one is now learning from these failures. It’s outrageous,” said Julian Hendy, the founder of Hundred Families, a charity that helps bereaved families. His own father, Philip, was fatally stabbed in Bristol in 2007 by a man who was seriously mentally ill.

“It means that lives are being put at risk because they aren’t learning from these deaths. There clearly could be increased threat to the public as a result of this,” he added.

The revelation that the Manchester team under Prof Louis Appleby have had to stop examining every killing comes days after Darren Pencille, who has paranoid schizophrenia, was jailed for life for murdering Lee Pomeroy on a train on 4 January. Pencille killed Pomeroy by stabbing him 18 times in 25 seconds in front of his 14-year-old son. A psychiatrist had deemed him to be no threat to himself or others the day before the attack.

Prof Sir Simon Wessely, the president of the Royal Society of Medicine, who reviewed the Mental Health Act last year at Theresa May’s request, said the decision was wrong.

“I think it’s a mistake to discontinue the central collection of homicide data. These events are fortunately rare, but of course an appalling tragedy to all those involved. But because they are rare it is essential that evidence is collected centrally, independently, and with the ability to learn lessons,” he said.

“These ghastly events have terrible costs, against which the costs of maintaining our current system of data collection and analysis are trivial,” added Wessely, a past president of the Royal College of Psychiatrists.

Mandatory investigation of killings by mentally ill people who had received NHS psychiatric care began in 1992. But NHS England stopped funding after deciding that that was no longer a top priority.

At the same time, it asked Appleby’s team to expand its examination of suicide, again to identify ways in which numbers could be reduced. As a result the project ceased to be called the National Confidential Inquiry into Suicide and Homicide in Mental Health and was renamed the National Confidential Inquiry into Suicide and Safety in Mental Health (NCISH).

In its last annual report the NCISH said: “We are reducing our homicide programme and in June 2018 we stopped sending questionnaires to clinicians for obtaining data on the characteristics of homicide offenders who had been in contact with mental health services.” The team now simply collates the number of such killings but no longer investigates them.

Dr Sarah Markham, a member of the Healthcare Quality Improvement Partnership’s advisory panel, recently criticised the decision. She wrote in the BMJ that: “My understanding is that the decision to cut the funding is due to wider budgetary pressures. However, given the extremely high costs incurred by treating a patient in a high secure psychiatric hospital – around £250,000 to £300,000 a year – it seems on economic grounds alone rather short-sighted not to fund work which can inform the prevention of homicide.”

The Royal College of Psychiatrists said it shared Hendy’s concern that ending routine investigation will leave the public more at risk. “The decision was very unwise. It is exceptionally important to be able to understand what has happened when people who came for treatment go on to kill,”, said Prof Pamela Taylor, the chair of the college’s forensic psychiatric faculty.

“In the longer term it is likely to affect public safety, because almost all of the homicide and other violence by people with mental disorder is likely to be preventable, providing we know enough about how the problems are related.”

NHS England has ended its funding despite the fact that it is spending record sums on mental health and even though such homicides often generate widespread public anxiety. Prof Tim Kendall, a consultant psychiatrist and NHS England’s clinical director for mental health, declined to respond to Hendy’s claim that stopping the work could increase the risk to the public.

“Every single death or homicide by someone in contact with NHS services is independently investigated, with the findings and lessons learned published online to help improve services as part of our long-term plan [for the NHS], which will boost mental health care by £2.3bn.

“These individual reports are alongside the 60 to 70 independent audits of NHS services, published openly every year, which look at NHS services as a whole and give a detailed, clinically led understanding of NHS care and how it can be improved.”