Poor communication has topped the complaints received by the Parliamentary And Health Service Ombudsman.
Complaints following inadequate apologies from the NHS have doubled as a proportion of complaints to 28% of all complaints received.
Complaints about clinical care and treatment, diagnosis and communication all accounted for around three out of ten complaints, while staff attitude accounts for two in ten complaints, including both behaviour and communication style.
The overall number of initial enquiries or complaints about separate NHS organisations received in 2013-14 was 18,870, of which 3,592 were accepted for investigation.
The number of complaints about separate trusts was 4,175 with 860 complaints accepted for investigation and 868 investigations completed.
There were wide varieties in complaints about NHS trusts, with some having a complaint referred to the ombudsman for every 250 received, while others the rate was one per 17.
Julie Mellor, parliamentary and health service ombudsman, said: "There are lots of reasons why numbers of complaints vary between hospitals and could be due to the size of the organisation or types of patients it serves, for example.
"We are publishing this data today because every complaint presents an opportunity to improve services.
"We know that poor communication, errors with diagnosis and poor care and treatment are the most common reasons why people complain to us about their hospital treatment. Other common reasons for complaints are staff attitude, no apology when things go wrong and unnecessary delay in treatment.
"We hope NHS leaders use the data in this report to identify themes, and recurring problems in order to understand what they have done well and how they can improve their complaint handling."
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