Elderly people are reluctant to make complaints about sub-standard health care or do not know how to and could be suffering in silence, a report by the Parliamentary and Health Service Ombudsman said.
The report found 56% of people over the age of 65 who had experienced a problem had not complained because they were worried about how it might impact their future treatment.
Almost one in five did not know how to raise a potential complaint, while about a third felt that complaining would not make any difference.
One carer from a focus group in Manchester told the authors: "When people have a problem they don't know where to go; they are referred to a computer which they don't have; they are referred to a library which is too far away to get to... (and) they wouldn't know what to do anyway."
The research was based on a national survey of almost 700 people over the age of 65, as well as focus groups and case studies.
Parliamentary and Health Service Ombudsman Julie Mellor said: "Older people are some of the most frequent and vulnerable users of health and social care services but are the silent majority when it comes to complaining."
She added: "Their reluctance to complain could mean that they are suffering in silence and could lead to missed opportunities to improve the service for others."
The research is a cause for concern and it is vital every hospital patient or health care client feels any potential complaints would be properly addressed, according to Age UK.
Charity director Caroline Abrahams said: "Patient feedback is a great barometer of the quality of care and this report suggests hospitals need to do much more to reassure older patients that they can complain if they need to, free from fear."
She added: "Seeking and responding to older people's views and experiences is crucial if we're to prevent future care scandals like those that have too often blighted our hospitals and care homes in recent years."
The report urged action, particularly due to Britain's ageing population. By 2030, around one in 10 people living in the UK will be 75 or over, according to the Office for National Statistics.
It recommended a more proactive approach from NHS providers, which need to make sure all clients are aware of how to complain and reassure them there would be no repercussions.
Commissioners of healthcare should also use the ombudsman's complaint-handling guidelines, My Expectations, as a way of measuring their own performance, the report recommended.
The report also pointed out progress was being made, including steps by the Government to explore options for a new streamlined public ombudsman service to handle complaints.
A universal, independent complaints advocacy service that was easy to find and simple to use would improve the situation, Healthwatch England said.
A spokesman said: "We know the NHS is under pressure at this time of year, it is therefore vital that if things do go wrong patients are informed how to raise concerns and how to get help to do so if they need it.
"Without this support, thousands of incidents will continue to go under the radar every year and mistakes will never be learnt from."
Healthwatch England said there was support available for those let down by the NHS but added: "When it comes to care homes and home care services there is little to no complaints support at all, leaving very vulnerable adults with little protection."