Patients are being let down by the way NHS trusts in England handle complaints, a watchdog has found.
Complaints from patients should lead to improvements in care
The Healthcare Commission found wide variation in the way complaints were dealt with - and a failure to act on any problems they threw up.
The audit focused on 32 trusts where concerns had been raised - 12 were found to have "significant lapses".
The NHS Confederation, which represents NHS bodies, said trusts welcomed feedback from patients.
The audit, the first of its type, covered primary care, hospital, ambulance and mental health trusts.
The Healthcare Commission reviews around 8,000 complaints a year in which a patient is dissatisfied with the response of the trust.
It found that nine trusts had failed to safeguard the care of patients who had registered complaints.
Overall, basic complaint processes were in place in all trusts - but too often the focus was on the procedure, rather than learning constructive lessons for the future.
The watchdog said trusts should do more to make it easier for people to raise a complaint.
Anna Walker, the commission's chief executive, said: "Given that the NHS provides 380 million treatments a year, the number of complaints (140,000) is relatively small.
"But when someone does complain, trusts need to respond well."
She added that if trusts did not learn from complaints it could "seriously damage" people's faith in the NHS.
The Department of Health has recently proposed a new system that will place greater responsibility on NHS organisations for ensuring that complaints are resolved locally - changes which could be in place within 12 months.
But the watchdog says its audit raises questions about whether trusts have the capacity and capability to manage complaints.
Ms Walker said: "There are serious questions about whether trusts are in a position to ramp up their systems in time to provide the necessary standard of service."
Action Against Medical Accidents said the way many NHS organisations handled complaints added "insult to injury".
Chief executive, Peter Walsh, said the government's proposals offered some hope for improvement.
"However, we are concerned that the loss of important safeguards such as having the right to an independent review of complaints by the Healthcare Commission could leave many complainants with nowhere to go."
Katherine Murphy, spokesperson for the Patients Association, said the audit findings mirrored what it had heard from patients.
"There needs to be more focus on treating patients as customers of the NHS and on customer care training - in the same way that big companies do."
Nigel Edwards, director of policy at the NHS Confederation, which represents more than 95% of the organisations that make up the NHS, said feedback from patients was invaluable.
"We stress that our members would never allow a complaint to adversely affect the quality of care they provide.
"However there is always room for improvement.
"Procedures may vary at local level but NHS managers will continue to listen to patients' views in order to deliver world-class healthcare."