The way the NHS deals with complaints is the most complained about issue
The NHS must do better at handling complaints, a health watchdog has said.
Complaints in England are initially dealt with by the NHS trusts, but can be referred up to the Healthcare Commission and then the ombudsman.
A report by the commission found the proportion of complaints it upheld rose by 50% last year compared with 2007.
The watchdog said this suggested NHS trusts were getting worse at dealing with them, but ministers said improvements were in the pipeline.
The NHS receives about 135,000 complaints a year - a figure which has remained stable over recent years.
About 9,000 of these cannot be resolved at a local level and are referred up to the Healthcare Commission.
Of these, the watchdog upheld 30% last year - up from 20% the previous year.
Another 17% of cases were sent back to individual trusts because their approach to tackling them was deemed insufficient.
This means nearly half of all complaints reviewed by the Healthcare Commission required further work by NHS trusts.
In less than one in five cases reviewed did the watchdog side with the trust, with the remainder either withdrawn, deemed outside the Healthcare Commission's jurisdiction or - in the most serious cases - passed on to the ombudsman.
The way the NHS handles complaints was also the number one issue raised by complainants, accounting for a fifth of cases reviewed.
Poor communication, standards of treatment and delay or failure to diagnose were the next most common themes.
Healthcare Commission chief executive Anna Walker said while it was encouraging that the number of complaints received remained relatively small considering the millions of treatments provided by the NHS, the service still needed to do better and learn from its mistakes.
"It is very important that people feel that they can complain about their NHS trust if they need to and that the trust will respond positively to their complaint and learn general lessons from it."
The watchdog published its report ahead of changes later this year which will see the system streamlined so that patients will go straight to the ombudsman if they are unhappy with the way the trust has dealt with the complaint.
This places even more emphasis on the trust to get its approach right as the ombudsman process is more expensive and time-consuming.
Joyce Robins, of Patient Concern, said the system was unclear and many trusts were not responsive enough.
"There's a body of trusts, and it's usually the same body of trusts, who don't do the job at all well," she added.
"The commission report says many of them fail to interview the staff involved in the complaints, they can't write a clear letter to the complainant - those are pretty simple things you would expect everyone to be doing."
But Health Minister Ann Keen said the changes would improve the system by making it more straightforward for patients.
However, she admitted some NHS bodies were not "handling complaints well enough", adding the regulation system was being changed to put more emphasis on the complaints procedure.
"Hospitals and other NHS organisations will be measured on how many complaints they receive and how they deal with them, including showing what lessons they have learned and changes implemented as a result."