An Oxfordshire care home's neglect of an elderly dementia patient who later died was not properly investigated by the county council, a watchdog said.
The woman became severely dehydrated and needed three weeks of hospital treatment after a stay at Huntercombe Hall Care Home, in Henley-on-Thames.
The Local Government Ombudsman said the private home and the council "failed to give the family proper answers".
Both organisations apologised and said they usually have "high standards".
The watchdog said the woman, who has not been named, died a week after being discharged from hospital.
It said her advanced dementia meant she struggled to swallow and could not say when she was hungry or thirsty.
Her husband said that when he picked her up from the care home she was "less responsive and limp" and her mouth had a coating of white spots.
Dr Jane Martin, the ombudsman, said the care home and the council both carried out investigations, but neither was "up to the standard I expect".
She said the care home found "no shortfalls in its treatment of the woman" and the council did not act in accordance with the law, government guidance or its own policies.
John Jackson, the county council's director of adult social services, said the way the case was handled was "very disappointing" but the council had made "robust and extensive improvements to procedures".
"This was an isolated case of poor practice and in no way systemic or representative of the general standards to which we operated in 2014 or now," he said.
A spokesperson for the care home, which is owned and operated by Caring Homes Healthcare Group Limited, said it would waive all fees for the woman's care.
A statement added: "We have subsequently implemented a range of measures at the home to further strengthen our monitoring and record keeping processes."
The county council has also been asked to pay the woman's husband £750 for his time, trouble and distress.