The number of care homes rated "inadequate" or requiring improvement increased in one in five English council areas last year, according to new analysis.
Figures compiled by older people's charity Independent Age showed dramatic variations in the quality of care homes across the country, with 28.2% performing poorly in the North West, compared to 17.4% in London and the East of England.
While the overall percentage of homes receiving the Care Quality Commission's bottom two ratings fell compared to the previous year, every English region had at least one local authority where the proportion rose in 2017.
Independent Age blamed low levels of funding from local authorities, poor pay and difficulties recruiting staff, as well as the lack of a good support mechanism for struggling homes to call upon.
Five of the eight worst-performing authorities on care home quality were in the North-West, said the charity. Some 26.1% of homes in Yorkshire and the Humber and 21.3% in the West Midlands received poor ratings.
Seven local authority areas had more than 40% of homes rated "inadequate" or "requires improvement": Tameside (56.8%), Portsmouth (46.5%), Kensington and Chelsea (45.5%), Manchester (43.8%), Bradford (43.6%), Stockport (43.3%) and Trafford (43.1%).
By contrast, no homes were rated inadequate or requiring improvement in Bracknell Forest, Isles of Scilly, Reading, Rutland or Southwark, and just 3% in Thurrock, 4.5% in West Berkshire and 4.7% in Richmond-upon-Thames.
Independent Age chief executive Janet Morrison said: "Older people and their families are still facing an unenviable choice between poor care homes in some parts of the country.
"While it is encouraging that there has been an overall improvement in quality, this masks persistent variation in the quality of care homes within each region of the country. The market simply does not seem to be able to drive the rapid improvement needed in many areas.
"While the Government seems happy to deflect all decisions about social care into the vague promise of a green paper, local authorities are having to make difficult decisions now about care in their area. We urgently need both Government and local authorities to demonstrate that they understand the reasons for this variation and that they have the ability to address it."
Dominic Carter, of the Alzheimer's Society, described the figures as "shocking and shameful" and said the 70% of care home residents with dementia were "at the mercy of a system on the brink of collapse".
"The Government must act now, with meaningful investment and a fresh approach to delivering care, or the system will collapse and people with dementia will continue to suffer needlessly," he said.
The chairwoman of the Local Government Association's Community Wellbeing Board, Izzi Seccombe said: "This study sadly reflects the consequences of the social care funding crisis and is further evidence of the ever more pressing need to adequately fund adult social care for both the immediate and long-term."
Labour social care spokeswoman Barbara Keeley said that £6.3 billion had been taken out of adult care budgets since 2010
"The unacceptable regional variation in the quality of care homes, which sees too many older people getting poor care, is the direct result of the Tories' devastating cuts to council budgets," she said.
But a Department of Health and Social Care spokesman said: "Eighty-one per cent of care services are rated as good or outstanding, and we're committed to driving further improvements and reducing variation.
"That's why we're consulting soon on new measures to do just that, and have provided local authorities with an extra £2 billion funding as well as a further £150 million for next year."
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