Four in 10 adults who have had concerns about the poor care of an elderly loved one admit they have held back from speaking up.
And more than two-thirds of those who did try and complain said it made no difference to the standard of treatment.
The survey of 600 middle- aged adults by the Parliamentary and Health Service Ombudsman also uncovered some harrowing examples of neglect.
One woman said her elderly husband had to call an external emergency number from a hospital floor after falling over and being unable to summon a doctor or nurse.
Others reported their relatives being "forgotten", "ignored" and even laughed at by nurses.
The Ombudsman said many older patients were "suffering in silence", partly because family members were too afraid to raise concerns.
Relatives often worry that making a complaint will only cause further trouble and lead to their loved one being treated even worse by staff.
Others believe the process is pointless and will not change anything, or do not know how to make a complaint.
The recently released research is based on a survey of 602 adults - mostly aged 50 or over.
They all had an elderly relative who had stayed in a hospital ward within the past 12 months, usually their parents or in-laws, but in some cases spouses.
The health service has repeatedly tried to overhaul its complaints system since a high-profile inquiry into the Mid Staffordshire hospital scandal in 2013.
That verdict warned of a "culture of fear" across the National Health Service (NHS), with patients, relatives and staff too afraid to speak up about poor care. Hospitals have since been told to make it far easier for people to complain by providing the relevant forms and contact details.
But this research suggests the efforts have had little impact, with campaigners saying there is still a "long way to go".
About 35% of those surveyed said there had been at least one occasion when they were concerned about their elderly loved one’s treatment.
But only 45% complained immediately, with a further 14% raising a concern afterwards, often after the patient had been discharged,
Another 40% did not complain at all as they either feared repercussions, did not want to make a fuss or weren’t sure how to go about it.
Of those who did try and complain, 67% said it made no difference to the standard of care.
The survey also found that 28% of relatives did not feel their loved one was given enough help with washing or eating.
A further 19% said patients weren’t treated with dignity or respect and 18% believed they were often in pain.
Most were treated on NHS wards but some were looked after in private hospitals.
Patients Association’s John Kell said the research showed that there was still a long way to go before the NHS’s complaints system was readily accessible and inspires confidence in patients, their carers and families.
“This seems particularly true in respect of older people,” he said.
Caroline Abrahams, charity director at Age UK, said the experience of being admitted to hospital was always an upsetting time for older people and their families, and difficulties of communication between families and hospital staff only add to what is a stressful experience.
“Everyone who goes into hospital should feel confident that they will be treated with dignity, and concerns or complaints about their care and treatment will be listened to and properly investigated."
The country’s Department of Health spokesperson said it was determined to make the NHS the safest healthcare system in the world and that when things do go wrong, it’s incredibly important to listen to patients’ and families’ complaints or wider feedback.
“By learning from mistakes we can improve care. This is why we made complaints handling a crucial element of the hospital inspection regime.
“These findings show more could be done to help older people and families complain; we are clear that organisations should be open about how to complain and clearly communicate the support available to people who need help in complaining."