The Care Quality Commission is being criticised for failing to take sufficient action after undercover filming showed an 80-year-old woman being slapped by a care worker.
Jane Worroll secretly filmed her mother's room in Ash Court, London.
She recorded care worker Jonathan Aquino slapping her mother six times.
Aquino was convicted of assault. The CQC said the home still "ensures people who use the service are protected from abuse or the risk of abuse".
Prior to the assault, the CQC had rated the home as "excellent" .
Alarm clock camera
Ms Worroll placed a secret camera hidden in an alarm clock in her mother's bedroom, after suspecting she was being mistreated.
Maria Worroll suffers from Alzheimers and arthritis, and requires around-the-clock care.
Six weeks after her mother moved into Ash Court, Ms Worroll noticed bruises on her arms and legs.
She secretly filmed her bedroom on 17 and 21 June 2011.
The footage, acquired by BBC Panorama, shows care workers feeding Maria Worroll too quickly, manually rolling her over and hauling her roughly into bed - an action that should have been performed using a hoist.
Ms Worroll also filmed one care worker, Jonathan Aquino, slapping her mother six times.
She was deeply upset by the footage. She told Panorama: "[My mother] is just so vulnerable; she can't get up, she can't call for help. [The assault] is just totally sadistic."
All five of the main carers Ms Worroll filmed were sacked. Jonathan Aquino was arrested and sentenced to 18 months in prison in April 2012.
Forest Healthcare, which manages the home, say the assault was an isolated incident, and that Ash Court is "committed to working closely with all families and residents" and that they receive positive feedback on their care, which is subject to "continual improvement".
The CQC visited the home twice following the assault to assess the quality of care. It concluded: "Ash Court ensures that people who use the service are protected from abuse, or the risk of abuse, and their rights are respected and upheld."
Ms Worroll said she felt let down by the inquiry. She told the BBC: "When I read [the report] it was just another slap around the face. I just felt like they'd basically given [Ash Court] a clean bill of health again, bar two minor adjustments.
"It makes me worry for other people who are potentially about to put a relative into a care home."
Judy Downey, head of the Relatives & Residents Association, a charity that supports care home residents and their families, also criticised the CQC report.
She said: "It doesn't say that a member of staff has been charged. It is not an honest document, it isn't a helpful document.
"This is really an area where the CQC can do so much if it uses its powers. They can actually cancel the provider's registration if they're shown not to be fit. We would suggest that that provider was very clearly shown not to be fit."
The CQC told Panorama in a statement that its report into Ash Court made clear a "serious incident" had taken place. It visited again recently, and said that it was "confident" it had "acted swiftly and correctly in light of the evidence" available.
A CQC spokesman said “CQC carries out an unannounced inspection of every care and nursing home in England every year – more often if we believe people may be at risk. This system of regulation can and does identify poor care which CQC then takes action to tackle.
‘However, what it cannot do is to identify and stamp out deliberately concealed abuse. By its very nature, concealed abuse takes place away from the eyes of managers and inspectors and can even take place, as in this case, in a well run care home. Abuse of vulnerable people is a criminal matter, and is rightly handled by the police and the courts.
“CQC has taken action against a number of providers where a current risk to people has been identified. In this case, the risk had been dealt with by the removal of the care staff involved by the home. CQC’s role was to make sure residents were protected once police and social services had acted to deal with the abuse shown in the hidden camera footage. CQC acted quickly and appropriately in this regard.
“It is the responsibility of the people who run and work in care homes to make sure they meet essential standards of care and to deliver good, safe care. CQC’s role is to check this through unannounced inspections and take action where we see poor care. Care staff, homes, councils, police and other stakeholders all have a part to play in the prevention of elder abuse. CQC also relies heavily on information from people who receive care and their families and friends. Anyone who sees evidence of abuse should call our whistleblower hotline.
“None of this detracts in any way from the appalling experience Maria Worroll had at this home. CQC welcomes the custodial sentence handed down to Jonathan Aquino. His behaviour was criminal abuse, and as such has rightly been handled robustly by the police and the courts. The sentence sends a strong message that it is unacceptable to abuse an elderly person in your care. We hope it will act as a powerful deterrent to others.”