Patients have found the complaints system unhelpful
Vital lessons which could be learned from complaints to the NHS are ignored, a public spending watchdog says.
The largest ever review of the system in England, by the National Audit Office, found "little evidence" that complaints improved services.
Most people dissatisfied with their care do not bother complaining formally, it said.
Ministers plan changes next year, but the committee chairman examining the report said it would be "no easy task".
Similarly-structured NHS complaints systems are in operation in England, Scotland, Wales and Northern Ireland.
They offer the chance that problems can be solved locally between the organisation and the person complaining, before independent reviews or ombudsman systems are called in.
However, these local systems have been dogged with criticism for years, with accusations that they take too long to deal with problems, are overly complicated for the public, and foster a defensive attitude among staff.
The National Audit Office (NAO) report findings suggest that all these problems remain in evidence.
It says that even when someone does successfully navigate the system, there is no guarantee that this will mean a better service for others.
Tim Burr, head of the NAO, said: "There is a lack of learning from complaints, and providers are not making clear to users that services are being improved as a result."
The report complained that there was no formal means for any "lessons learned" from complaints to be recorded so they could be shared with others.
Approximately 19 out of 20 complaints are concluded "locally", and three-quarters of these are dealt with within 25 working days.
NHS complaints which need to be reviewed by the independent Healthcare Commission take an average of 171 working days.
'Defensive and unhelpful'
Edward Leigh MP, the chairman of the House of Commons Public Accounts Committee, which oversees the work of the NAO, said that the system was no way to keep people's "faith and trust" in health and social care services.
"Complainants are often confronted with a defensive and unhelpful response when sometimes all that is needed is a simple apology or a promise to improve services.
"Today's report indicates that it will be no easy task to achieve a properly functioning system which gives the public confidence that complaints will be dealt with effectively and fairly."
Marcia Fry of the Healthcare Commission, the NHS watchdog, said: "Too often NHS trusts do not systematically learn the lessons from complaints.
"And our work has shown that most people who complain about the NHS want the matter to be resolved quickly and in a non-defensive way."
A spokesman for the Patients' Association said the NHS was "light years away" from a genuine complaints service.
"For far too many patients it is no 'service' at all. It lacks compassion, is bureaucratic beyond belief and takes far too long. This is not a 'service', it is a sham."
The Department of Health said it had significantly improved the speed with which the NHS dealt with complaints over the past year, but agreed that more needed to be done to ensure that lessons were learned from them.
A spokesman said: "From next year, we are simplifying the system. Greater emphasis is to be placed on working with the complainant to resolve cases satisfactorily at a local level.
"We are also requiring NHS organisations and local authorities to publicise the complaints procedure and encourage people to use it.
"In addition, local organisations will publish information on the number of complaints received and how they have been dealt with."