Over half of care homes in some parts of England are rated Inadequate or Requires Improvement, according to Which? analysis of CQC data.
The analysis revealed that six local authorities have 50% or more failing homes.
Seven in 10 (69%) care homes are rated Requires Improvement or Inadequate in the London Borough of Westminster.
The ratio was three in five in Manchester and Wakefield (58%), followed by Kirklees (57%), Portsmouth (56%) and Tameside (55%).
In 45 local authority areas, a third of care home places were in poor quality homes.
Nine of these councils were in London, including Tower Hamlets (48%), Islington (47%), Kensington and Chelsea (46%), Newham (41%), Haringey (41%), Barnet (40%), Ealing (35%) and Harrow (33%).
On the plus side, a small number of areas had local authorities where nine out of 10 care homes were Good or Outstanding.
These included: Isles of Scilly (100%), Richmond Upon Thames (94%), Rutland (91%) and Blackburn with Darwen (90%).
Which? warned that the situation of failing care homes could get worse with its previous analysis revealing that almost nine in 10 councils could see a shortfall in places by 2022.
The consumer champion added that the research raised questions about whether councils could continue to honour their statutory duty to offer residents at least one care home that met their needs.
Martin Green, CEO of Care England, told us: “The Which data adds to growing mountain of evidence that social care chronically underfunded and this is starting to have a serious impact on both the quality, and the viability of services.
“If we are going to have a sector that is fit for purpose in the 21st-century, we need a clear and long-term strategy on how care is funded, and social care needs to be treated as a part of national infrastructure, and given the same priority as the NHS.”