Record numbers of complaints have been logged at NHS hospitals and community health services in England over the past two years.
The annual rise in complaints is also the highest since records began 12 years ago, according to the NHS Information Centre.
Between 2008-09 and 2009-10, the number of complaints rose from 89,139 to 101,077, representing a 13.4% rise and the biggest year-on-year increase since 1997-98.
The data refers to the number of written complaints about NHS hospital and community services.
Previously, the biggest yearly rise was 10.6% between 1999/2000 and 2000/01. Since 1997/98, there has been an average annual increase of 1.1%.
The report showed that, in 2009/10, the highest number of complaints (44.2%, or 44,682) related to the medical profession, a similar proportion to the previous year. The second highest proportion related to nurses, midwives and health visitors (22%, or 22,203), while NHS trust administrative staff accounted for 9%, or 8,635.
When broken down by topic, the biggest percentage of complaints related to "all aspects of clinical treatment" (42.2%, or 42,727 complaints), followed by staff attitude (12.2%, or 12,331), and delays or cancellations to outpatient appointments (10.6%, or 10,710).
All NHS trusts and community services are required to supply data for the report but it is not compulsory for foundation trusts to do so. There are 130 foundation trusts, of which 18 did not supply any data for the study.
NHS Information Centre chief executive Tim Straughan said: "This report shows the biggest annual rise in written complaints about NHS hospitals and community services for 12 years. However, it is important to bear in mind that there has been a substantial increase in NHS activity in England over time. For example, information from two of our other data collections show that hospital admissions increased by 28% between 1998/99 and 2008/09, while GP consultations increased by an estimated 44% between 1998 and 2008."
Health Minister Paul Burstow said: "These figures mark the last 12 months of the Labour Government's stewardship of the NHS. Despite the hard work of NHS staff, they demonstrate that, after 13 years of targets and tickboxes, more people than ever complained about their experience of the NHS. The coalition Government is determined to put patients at the centre of everything the NHS does."