NHS hospitals cleared their staff of blame in almost three quarters of "avoidable" patient death and serious harm cases where failures by medics were later independently discovered, a new report reveals.
Internal probes into such cases were also "not good enough" because they were not "consistent, reliable or transparent", Health Service Ombudsman Dame Julie Mellor said.
Dame Julie, who produced the report, also accused NHS trusts of putting up a "wall of silence" facing families of those who died or were harmed when they questioned how it happened.
She said: "Our review found that NHS investigations into complaints about avoidable death and harm are simply not good enough. They are not consistent, reliable or transparent, which means that too many people are being forced to bring their complaint to us to get it resolved.
"We want the NHS to introduce an accredited training programme for staff carrying out these investigations as well as guidance on how they should be done, so the public can be confident that when someone is needlessly harmed it has been thoroughly investigated and answers provided, so that action can be taken to prevent the same mistakes from happening again."
The report, A Review Into The Quality Of NHS Complaints Investigations Where Serious Or Avoidable Harm Has Been Alleged, was launched after the ombudsman found a wide variation in the quality of investigations carried out by the NHS into complaints about avoidable death and harm, it said.
It was based on interviews with hospital staff, a survey of NHS complaint managers and a review of unresolved NHS complaints brought to it. It looked at 150 complaints and found that 40 internal hospital probes were "not adequate".
It noted: "In nearly three quarters (73%) of cases where the Parliamentary and Health Service Ombudsman found clear failings, hospitals claimed in their earlier investigations of the same incident that they hadn't found any failings."
More than half (52%) of internal investigations where a clinician reviewed what had happened were carried out by medics "not independent of the events complained about", the review discovered.
Hospitals failed to class more than two thirds (20 of 28) of avoidable harm cases as serious incidents, meaning that they were not properly investigated, it also found.
Almost a fifth (19%) of NHS investigations were missing crucial evidence, such as medical records, statements, and interviews, and more than a third (36%) of internal probes which recorded failings did not find out why they had happened, despite nine out of ten complaint managers saying they were confident they could find answers.
It also found that even where care failures were discovered trusts did not always take steps to stop the same mistakes reoccurring.
Senior managers were blamed for not discussing possible improvements with frontline staff. The report also found no consistency of training for NHS investigators.
The ombudsman made five recommendations to the Independent Patient Safety Investigation Service (IPSIS), which launches in April. They include better organisation and consistent standard of investigations and making hospital trusts have "clear objectives" for staff to be "open and honest".
Neil Churchill, NHS England's director for improving patient experience, said: "When people make a complaint that they have been seriously harmed, they should expect it to be taken seriously and thoroughly investigated.
"The best way to improve care is to listen to what patients and their families tell us and learn from past experiences.
"Good quality, timely and consistent investigations are vital and the NHS continues to work hard to ensure patients feel confident raising issues or concerns."