Social care is facing a funding gap of more than £1bn by 2014 in England - a situation which would have consequences for the NHS, a leading think-tank says.
The King's Fund analysis predicted councils would struggle to protect home help and care home places as they come to terms with funding cuts.
The report said if this happened there could be more admissions to hospital and longer delays in discharging.
But the government said it did not believe there would be a funding gap.
The Department of Health has earmarked more money for social care.
By 2014 there will be an extra £1bn made available to local authorities from the NHS budget. The Department of Health allocations to councils are also rising.
However, local authorities also rely on funding from other government departments, which will be cut by 27% over the next four years, as well as the money they can raise from council tax to pay for social care.
The King's Fund said the most likely scenario was that the social care budget would be cut by 7% in real terms - meaning a funding gap of £1.2bn would open up within four years.
If this happened it would mean fewer people getting help at home or access to care homes.
This, in turn, the report argued, would increase the risk of emergency admissions because of accidents such as falls or deteriorating health as people struggled on with less support from social services.
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The lack of support could also mean a rise in delayed discharges - whereby hospitals cannot release people because of a lack of help available when they return home.
The King's Fund said one solution to the problem could be the creation of a joint settlement for NHS and social care and the pooling of budgets between NHS trusts and councils.
Report author Richard Humphries said: "The vital role played by social care in supporting the NHS to meet people's needs is well known.
"However, health and care needs are in fact two sides of the same coin - social care should no longer be viewed as just a supportive handmaiden to the NHS."
Care services minister Paul Burstow said there would be no funding gap as the extra money coupled with more efficient services would ensure those in need got help.
"We do not recognise the Kings Fund's figures," he said.
"The government recently allocated extra money to encourage more joined-up working, support the delivery of social care and protect the most vulnerable in society.
"This funding should enable local authorities to protect people's access to services and deliver new approaches to improve their care."
Councillor David Rogers, of the Local Government Association, agreed councils were facing a shortfall in funding and social care was being hit but tough decisions were "unavoidable".
He added councils would be doing everything possible for the most vulnerable people.
"Residents see social care as a priority front line service, and so do the town halls who serve them."