A series of shocking incidents in privately run residential nursing homes in Cornwall have been secretly filmed by the BBC.
The Panorama undercover filming shows one nurse saying she will give morphine to a resident "to shut her up" - an event described by an independent expert as "horrifying".
One of the homes, Clinton House in St Austell, is being closed.
The Panorama team raised safeguarding concerns with the local authority.
The Morleigh Group, which runs the homes, said it had already identified problems, and that it had always acted quickly to improve care when asked to do so.
The footage was recorded by three reporters - one posed as a resident, the other two got jobs there.
Some of the most serious incidents occurred at Clinton House, which has 32 residents. It was given a "requires improvement rating" by the Care Quality Commission in May 2016.
One of the Panorama team checked in as a resident in need of respite care.
What Panorama's undercover 'resident' saw
Janice Finch posed as a resident at Clinton House and St Theresa's - both run by the Morleigh Group, which is owned by Steven and Patricia Juleff.
"The first thing I noticed at Clinton House was the smell - an overpowering odour of urine.
"I'm not sure if it was the pressure cushion or the damp chair I was sitting on.
"Welcome to life in a British nursing home.
"What I witnessed in both homes was staff run off their feet, trying to cope with the demands of many immobile residents.
"A visit to the toilet requires two care assistants to work the hoists that lift a person out of their chair and into a wheelchair.
"But try and find two free care assistants, especially at meal times.
"I watched one lady wait so long to be taken to the toilet that she later confided in me that she had wet herself in the meantime.
"I spent two weeks in Cornwall, one of the prettiest parts of England. But this was no holiday camp.
"My whole experience raises serious questions."
Another Panorama team member, called Lucy, was hired as a care assistant at Clinton House.
The home claims its staff are qualified to look after people with mild dementia.
But during her time there, Lucy was asked to look after a resident with severe dementia - despite having had no specialist training.
Using a hidden camera, she filmed the resident lying in a bed with a loose guard rail.
When she raised her concern that the lady might fall out or trap a limb, another member of staff advised her that his solution was to hold his foot against the bed frame.
The Morleigh Group is understood to have been paid about £3,000 per week to care for the resident.
Lucy also witnessed an out-of-date prescription supplement being re-labelled and issued to a patient for whom it had not been originally prescribed.
On another occasion, she was asked to help care for a resident who was suffering from a degenerative disease.
She was prescribed morphine "as required". The resident said she was feeling unwell, and was worried she would need to go to hospital.
Lucy went to the nursing station to report what she had been told to a colleague.
The nurse she consulted appeared to become angry, and proposed giving the resident a dose of morphine, which, she said, "will shut her up".
Panorama showed its footage of the incident to Dr Peter Holden, a GP and member of the British Medical Association.
"I'm horrified," he said. "Morphine is not there to be a cosh on patients. That won't have been prescribed for that and would never be so."
As well as announcing the closure of Clinton House, the Morleigh Group has suspended the nurse pending an investigation.
The group denies that it failed to give proper attention to the quality and safety of residents' care.
It says it it had already dismissed the home's manager before it became aware of Panorama's investigation.
In a statement, Cornwall Council - which placed residents at Clinton House - said it was "sorry that the standard of care provided by Morleigh Group has fallen far short of what residents, their relatives and the general public have a right to expect".
It added that it was now investigating two more Morleigh Group homes.
The regulator, the Care Quality Commission, said it has had serious concerns with the Morleigh Group for some time, and had issued warning notices.
Andrea Sutcliffe, the CQC's chief inspector of adult social care, said: "We were appalled to have found that the Morleigh Group has allowed the quality of care to decline."
Panorama: Nursing Homes Under Cover will be broadcast at 20.30 GMT on BBC One on Monday 21 November. If you miss it you can catch up online.