Spot checks will be carried out at NHS trusts in England if there are complaints that staff are failing to meet standards of elderly care.
The preservation of dignity is seen as a crucial part of care
The Healthcare Commission issued the warning after inspections at 23 trusts which included interviews with staff and ward observations.
It comes amid mounting anxiety about the welfare of older patients.
Mixed-sex wards, lack of help with eating and being left in soiled clothes are the issues of particular concern.
The 23 trusts were chosen due to concerns over levels of care.
The watchdog found no major breaches of national standards, but only five of the trusts were found to have fully complied with all the standards relating to dignity, privacy and nutrition.
West Dorset General Hospitals
West Hertfordshire Hospitals
Queen Elizabeth Hospital
Luton and Dunstable Hospital
Oxford Radcliffe Hospitals
The Princess Alexandra Hospital
Hull and East Yorkshire Hospitals
Barts and The London
Eight trusts were warned they were at risk of not meeting the standards and would be scrutinised carefully again next year. A further ten were told to make a series of improvements.
The commission warned all hospitals that should "clusters of evidence" emerge, unannounced visits would be made.
"Where there is evidence that the right care is not being provided, we will use all our powers of assessment and inspection," said chief executive Anna Walker.
"Patients and the public do not want us to let go of this issue and we have no intention of doing so."
Provision of single sex accommodation is a key part of providing privacy to older patients but the commission found many trusts were struggling to provide this due to pressure on beds and the mix of patients.
Patients would only be put in a mixed-sex ward if they had agreed but the commission noted the issue of consent in the case of critically ill patients was a thorny one.
Proper nutrition is also widely held to be crucial in the treatment of older people and there are continual complaints that nurses do not have the time to make sure a patient has been able to eat the meal on offer.
The wishes of older people are often disregarded in the delivery of care, despite having expressed their wishes at the time of admission
While the commission found that there were improvements in both the meals on offer and providing patients with assistance in eating, it said almost half of the trusts inspected still needed to do more.
There was patchy implementation of policies regarding mealtimes, for instance, and a lack of formal arrangements to identify those who needed help eating.
One of the most distressing complaints reported was that patients were being left in soiled clothes, were being asked to wear a nappy and were suffering accidents because of a lack of assistance to get to the toilet.
However, the commission said it had found during its inspections that patients were being given assistance in using the toilet.
Nonetheless, it did find that in some cases locks were not always on the door and that toilets were being used by both sexes.
And when it came to the general delivery of care, the commission said it was concerned that the wishes of patients were "often disregarded".
Help the Aged said the report showed there has been "some progress" in recent years.
"But only five of the 23 trusts complied with all dignity in care standards," said policy director Paul Cann. "This is intolerable."
The British Geriatrics Society said the report had not spent enough time on the issue of personal care and going to the toilet.
"We can no longer allow frail older people to be told, 'do it in your pants', or be left soiled for hours at a time."
Liberal Democrat health spokesman Norman Lamb said many older patients feared going into hospital.
He said the government had failed on act on a pledge to end mixed sex wards.
"Nurses would have more time to deal with the individual needs of patients... if they didn't have to chase the endless targets set by ministers in Whitehall," he added.
Minister for Social Care Ivan Lewis said: "Dignity and respect should be at the heart of services for older people.
"The test for all frontline staff should be whether they would be happy with the standard of care for a member of their own family."
The commission's report echoed many of the problems listed recently by MPs and peers in a report on standards of care for older people in hospitals and care homes.