The number of people demanding their NHS complaint be independently reviewed has more then doubled in a year, new figures show.
The commission is currently investigating 4,500 complaints
The Healthcare Commission, the watchdog which handles complaints unresolved by local trusts, received 8,000 requests in the 12 months to August.
That compared to 3,700 independent review requests in 2003/2004 when the NHS dealt with second stage complaints.
The commission is urging the NHS to resolve more cases at local level.
It also called on NHS trusts to provide important information more quickly.
The commission is currently sending one in three cases back to the NHS because the complaint has not been dealt with well enough, the figures show.
That figure had increased from 27% of complaints by the end of May to 32% at the end of September.
The commission's head of operational development Marcia Fry said the rise in the number of complaints referred for independent review was "dramatic".
"We have been working as hard as we can to get as many NHS complaints resolved as quickly as possible and those efforts are now bearing fruit," she said.
"However, all trusts must also play their part."
NHS trusts must improve their ability to resolve complaints from patients "quickly and locally", she added.
"It is worrying that so many of the NHS complaints that come to us are having to go back to the NHS to be put right."
The review process was sometimes slowed down because trusts failed to send complaints information.
Figures show that 4,500 complaints are currently being investigated by the commission with about 1,600 being with them for more than six months.
The commission was still waiting for more information from NHS providers in 25% of those cases.