Half of England's large care home providers for the elderly have at least one in four failing homes in their network, a study has found.
Analysis of Care Quality Commission (CQC) inspection results showed some of the 54 providers operating at least 12 homes in England had half or more of their premises classed as failing by the regulator, Which? found.
Two of the biggest, Four Seasons Group and Orchard Care Homes, finished near the bottom of the rankings.
More than a third (35%) of 167 Four Seasons homes included in the analysis were rated as inadequate or requiring improvement by the CQC, while almost half (46%) of 44 Orchard homes did not meet the required standards.
The worst performer, Ideal Care Homes, which runs 16 homes across the Midlands, the North West and Yorkshire, had more than three in five (63%) classed as failing by the regulator.
It was joined in the bottom three by Akari Care Ltd, which had 60% of its 20 homes included in the analysis given poor ratings, and Derbyshire County Council, which had 10 failing homes (50%).
However at the top of the table, both Avery, which operates nationwide, and North Yorkshire County Council, had all their homes rated as good or outstanding.
Just 13% of English homes with 10 beds or less were rated as inadequate or requiring improvement, with this rising to 41% among those with 120 beds or more.
Which? has warned that the situation could deteriorate rapidly as demand for places starts to outstrip supply in some areas.
Previous Which? research has found that almost nine in 10 council areas across England are facing a shortfall in care home places by 2022, with some of those areas already having more than half their beds rated as requiring improvement or inadequate.
Which? is calling on the Government to act urgently on the Competition and Markets Authority's recommendations on information provision, complaints and unfair charges, and to use its Green Paper to set out how it will deliver a sustainable system that provides affordable, high quality care for all.
Alex Hayman (pictured), Which? managing director of public markets, said: "Too few providers are able to offer consistent, high quality care, limiting choice for stressed families in a system which is already close to breaking point.
"The Government must now ensure that its Green Paper on care delivers the fundamental reforms needed to secure high quality, affordable care for older people - both now and in the future."
Four Seasons Health Care said: "65% of our homes in England are rated as 'good' or better, which is 20 percentage points higher than two years ago.
"When CQC inspectors rate a home as requiring improvement, it means that they have found some things it needs to do better, but it doesn't mean the home overall is failing and it doesn't follow that residents are dissatisfied."
Izzi Seccombe, chairwoman of the Local Government Association's community wellbeing board, said: "This study sadly reflects the harsh reality of the social care crisis, which is affecting the quality of care provided and its availability.
"It is a further warning on the sustainability of the care market, which is becoming increasingly fragile.
"There is an urgent need for genuinely new funding and long-term reform of the sector if councils and providers are to address the severity of challenges facing the provider market to ensure people receive high quality care at the right time and in the right place for them."
Copyright (c) Press Association Ltd. 2018, All Rights Reserved.