Some 50 per cent of beds - or more - are in homes rated as needing improvement or inadequate in six local authority areas, Which? analysis of Care Quality Commission (CQC) data has found.
The lack of good quality care is particularly acute in the London borough of Westminster, where seven in 10 beds (69 per cent) were found in care homes rated as below standard. In Manchester and Wakefield, three in five beds (58 per cent) are in below-standard homes, closely followed by Kirklees (57 per cent), Portsmouth (56 per cent) and Tameside (55 per cent). In total, nearly a third of local authority areas, or 45 councils, have one in three beds or more in poor-quality care homes.
Nine of these are in the capital and include Tower Hamlets (48 per cent), Islington (47 per cent), Kensington and Chelsea (46 per cent), Newham (41 per cent), Haringey (41 per cent), Barnet (40 per cent), Ealing (35 per cent) and Harrow (33 per cent).
At least nine in 10 care home beds are in homes rated as good or outstanding in a small number of areas, including the Isles of Scilly (100 per cent) Richmond upon Thames (94 per cent), Rutland (91 per cent) and Blackburn with Darwen (90 per cent).
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Which? said the analysis highlighted the "huge" regional variation in the provision of quality local care across the country.
The watchdog warned that the situation could rapidly worsen as demand starts to outstrip supply in an increasing number of local areas.
Previous Which? research found that almost nine in 10 council areas across England were predicting a shortfall in care home places by 2022.
The Competition and Market Authority (CMA) is currently investigating the sector with a focus on market-wide issues including provision.
The Government announced recently that it would publish a Green Paper on social care next summer, and Which? is calling for it to look at the care system as a whole.
The Competition and Markets Authority is expected to release its final report into the care home market later this week.
Alex Hayman, Which? managing director of public markets, said: "Having to choose a poor care home isn't really making a choice at all, and it's disturbing to know that so many people across the country are already in care homes that are clearly not good enough.
"The Government must use its Green Paper to tackle the very real issues in care, including quality, provision and choice, before the situation gets much worse."
Izzi Seccombe, chairwoman of the Local Government Association's community wellbeing board, said: "These findings show that the social care crisis is very real and that already we are seeing the consequences of the chronic underfunding of the system on providers and the quality of care.
"The announcement of a social care Green Paper next summer will be important in delivering long-term reform, but older and disabled people who need care and support right now cannot be left to make do with sub-standard quality until then. Urgent action is needed now."
A Department of Health spokesman said: "It is unacceptable that standards fall below those rightly expected by people in care and their families.
"That's why we introduced a comprehensive inspection regime which has rated 80 per cent of adult social care providers as good or outstanding and changed the law placing a duty on councils in England to offer a meaningful choice of services.
"We know there's more to do so have provided an extra £2 billion of funding over the next three years and next summer we will publish plans to reform social care to ensure it is sustainable for the future."
A Westminster City Council spokesman said the authority was "proud of the quality of its adult social care provision" but recognised that "more can be done".
He added: "Westminster City Council has recently agreed to additional funding for targeted work to improve care homes in Westminster."