English councils face a deficit of £1.77 billion in their social care budgets for 2005/06, a survey of social services departments has found.
An ageing population is increasing demand
It blames the NHS financial crisis, under-funding from central government and an ageing population.
As a result councils say seven out of 10 adults will only receive support if their needs are critical.
The Association of Directors of Social Services and the Local Government Association carried out the survey.
It found eight out 10 councils say they are tightening eligibility criteria for access to services for people with disability.
A similar number - 77% - are adopting the same strategy for older people.
The report warned that an ageing population will continue to demand services at an unprecedented level.
By 2007 it is forecast there will be more people over 65 than under 16 for the first time, and in many areas this is already the case.
The report also highlighted the local government finance settlement, which gave half of councils with social services responsibilities a 2% or less increase - below the rate of inflation.
The financial problems of the NHS were also blamed for increasing the number of people receiving social care, rather than care in a hospital.
Latest estimates predict the NHS could run up a record deficit of around £800 million for the current financial year.
In addition, the report flagged up big increases in the cost of contracts with independent and voluntary sector providers.
It said average increases were 4.2% for nursing and residential care, and 4% for home care.
Councillor David Rogers, Local Government Association social care spokesperson, stressed councils planned to make £220 million of efficiencies in social care over the next financial year.
He said: "Councils are leading the way in making efficiency savings and delivering ever better value for money in the public sector.
"But the increased number of people in need of care, coupled with some extreme financial pressures, has left many authorities with some deeply worrying problems.
"Health and social care are two sides of the same coin. Underinvest in one and you over stretch the other.
"The number of people in need of care is increasing by the day and a fair, sustainable system of funding must be introduced."
Julie Jones, president of the Association of Directors of Social Services, said her members had expressed growing concerns about cost pressures across adult social care and children's services.
She said social services departments had been asked by the government to invest in prevention and early intervention strategies to cut abuse of the vulnerable - and were keen to do so.
"But with budgets so stretched by responding to the needs of those with high care requirements it is difficult to see how we can make the shift without additional resources."
Care Services Minister Liam Byrne said: "As part of our preparations for the 2007 Comprehensive Spending Review, the Department of Health has set up a working group with the ADSS, LGA and other local government and voluntary sector organisations to evaluate a range of issues, including funding."