The NHS ombudsman - the independent service that investigates patients' complaints - is "wholly ineffective and failing families", warns a charity.
The Patients Association says it receives weekly calls from people who feel let down by the service.
And they all said the experience compounded the grief and hurt they were already feeling through loss of loved ones or due to poor NHS care.
The ombudsman said it was improving its service.
The Parliamentary and Health Service Ombudsman (PHSO) is meant to be the final arbitrator for complaints about the NHS in England.
The Patients Association says it has lost faith in the service and no longer advises callers who ring its national helpline to go to the PHSO.
"As one of our recent callers said, 'You may as well ask a poacher to investigate the missing pheasants,'" says the charity in its report on the topic.
Prolonged investigations, which rely on families to produce all the evidence, can lead to patients or their families having to give up their employment to deal with the demands and inadequacies of the PHSO, it says.
Earlier this year the Patients Association supported the parents of Sam Morrish, a three-year-old boy from Devon, who tragically died through a catalogue of NHS failures of care. Sam died in 2010. It took until 2014 for the PHSO to pronounce its findings.
The ombudsman acknowledged that it had taken far too long to investigate the case.
How to complain
Stage One: Make a formal complaint to your service provider such as your GP, dentist, hospital or pharmacist in writing
Stage Two: If you are unhappy with the outcome of your complaint, you can refer the matter to the Parliamentary and Health Service Ombudsman
Source: NHS Choices
Patients Association chief executive Katherine Murphy said: "We wish we could say cases like that of Sam Morrish and those other families are a once-in-a-lifetime situation, but they are not.
"The PHSO cost to the public purse is around £40m a year, but we have no idea how it really does its job.
"The emotional cost for families far outweighs the huge financial cost."
The Patients Association wants to see an overhaul of the PHSO.
A PHSO spokeswoman said: "We have embarked on a radical modernisation drive which includes listening to feedback from users."
She said the service was investigating six times more complaints than the previous year, yet had maintained satisfaction levels.
"We are delighted that the Patients Association 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.