Almost seven million people over the last five years have had concerns about their care but have never raised them, according to a regulator.
The Care Quality Commission (CQC), which inspects organisations including care homes, GP surgeries and hospitals, said most people feel regret if they do not raise their concerns.
It is launching a new campaign – Declare Your Care – to urge people to tell it about poor care as a way of improving services.
A poll for the CQC of 2,002 people found 16% had wanted to complain in the last five years but did not, while 30% did complain and 54% had no complaints.
The most common reasons for not raising a concern were not knowing how to (20%), not knowing who to address concerns to (33%) and not wanting to be seen as a troublemaker (33%).
Some 28% of people were worried about not being taken seriously, while more than a third (37%) believed nothing would change as a result.
However, when people did raise a concern or complaint, the majority (66%) found their issue was resolved quickly, it helped the service to improve and they were happy with the outcome, the CQC said.
The three main reasons why people complain – or why they wish to – are delays to a service or appointment, lack of information and poor patient care.
More than 20% also have worries about the lack of communication between health and care services.
Ian Trenholm, chief executive at the CQC, said most people are getting good care, but added: “We know that when people raise a concern they have a genuine desire to improve the service for themselves and others.
“We also know that the majority of services really appreciate this feedback and make positive changes, as this new research shows.
“Hearing from people about their experiences of care is an important part of our inspection work and contributes to driving improvements in standards of care.
“Everyone can play a part in improving care by directly giving feedback to services, or by sharing information and experiences with us so that we can take action when we find poor care.”
Care minister, Caroline Dinenage, said: “We want the NHS and social care system to provide the safest, most compassionate care in the world.
“This means encouraging patients to speak up with concerns, ensuring we act on them and learning from what happened so we can do better in future.”
People can share their own experiences or those of a family member at www.cqc.org.uk/sye