The chances of finding a good care home in some areas of England are close to impossible, an analysis suggests.
The review, by the Institute for Public Policy Research, found high concentrations of under-performing care homes in a number of local areas.
In a sixth of places over a third of beds for the elderly and disabled were in settings rated as not good enough.
Campaigners said this risked leaving families with no choice but to accept substandard homes for relatives.
In each local authority area the IPPR think tank mapped the proportion of care home places rated as inadequate or requiring improvement by the Care Quality Commission.
Overall, 23% of the 456,000 beds were in settings that fell into one of the two categories.
The best and the worst
But in some places the concentrations were so high that half or more of beds were in under-performing homes.
The London borough of Newham had the highest concentration – more than 60% of beds were in homes judged not good enough. This was followed by Manchester on 50%.
Overall, 26 local authorities had more than a third of beds in sub-standard settings.
By comparison, Southampton, Windsor and Maidenhead, Peterborough and Kingston, in London, all had less than 5% of beds in homes judged not good enough.
Caroline Abrahams, of Age UK, said the situation was “unacceptable” and left some people with “no choice” but to accept sub-standard care homes because good homes would be over-subscribed.
“The sad truth is that following years of underfunding and neglect, the social care system has now more or less completely broken down in some parts of the country,” she said.
The IPPR agreed it was a sign of a failing market.
The think tank said costs had been cut in the care sector, and urged the government to take action by investing and reforming social care.
It wants to see tax rises to provide free personal care for all and to help expand the number of beds by up to 75,000, arguing the state could be more involved in running services.
Care companies have complained over recent years they have been squeezed by the low fees paid by councils, which fund most care home places.
The system is means-tested for over-65s, which means there are large numbers of self-funders.
The government said its plans to reform the system would be published in “due course” and pointed out it had announced an extra £1.5bn for the sector for 2020-21 in the recent spending round.
“We expect everyone to be able to access high quality, safe and compassionate care,” a Department of Health and Social Care spokesman added.