The NHS ombudsman – the body charged with investigating complaints that people have been treated unfairly or have received poor service – is "unaccountable" and "wholly ineffective", according to the Patients Association (PA).
The charity warned that families who have turned to the Parliamentary and Health Service Ombudsman (PHSO) feel "stonewalled" by the organisation because of its " unacceptably flawed and out dated" investigation processes.
The PA cited the case of Sam Morrish, a three-year-old boy who died from sepsis poisoning and a series of NHS failures of care.
Morrish passed away in December 2010 but it was not until 2014 that the PHSO published its findings.
However, the organisation did recognise that it took "too long" to investigate the case.
"The PHSO should be a court of last resort where uncorrected mistakes by the NHS can finally be put right, but the process is not fit for purpose and often ends up compounding the grief of families," said Katherine Murphy, the chief executive of the PA.
"The quality, accuracy, objectivity, effectives, openness and honesty of its reports is shameful."
The PA also claimed that it has evidence of "serious failings" of the PHSO and said it is time to hold the body to account.
The PA called on the Public Administration Select Committee and the UK government to bring about that radical change and give the public confidence that complaints will be fully and professionally investigated.
A PHSO spokesperson said: "We have embarked on a radical modernisation drive which includes listening to feedback from users."
"We are delighted that the PA has agreed to help us draw up a service charter, which will be a set of promises to users about what they can expect when they use our service," she added.