Major changes to the social care system which come into place in England today are "built on sand" unless more funding is allocated, charities have warned.
The Care and Support Alliance (CSA), a coalition of more than 80 charities, said the Care Act is a "bold attempt" to reform the system, but with an estimated £4.3 billion funding black hole for social care services by the end of the decade, thousands of disabled and older people risk losing out on vital help.
The act, which was passed last year, is the most comprehensive overhaul of the system since 1948, providing the first-ever national eligibility threshold - a set of criteria determining when local authorities will have to provide people with support, which is aimed at tackling the variations between local authorities.
Reforms also include a personal cap on personal care costs of £72,000, excluding accommodation, and councils will have a new duty to provide preventative services.
Care and Support Minister Norman Lamb previously said putting people in control of their care and limiting the amount anyone may have to pay for the support they need would make the system fairer.
But the measures have been widely criticised by age and health charities as well as companies providing care services.
The CSA said the new threshold will mean many people whose needs are not deemed as serious will no longer be eligible for help, while chronic underfunding is only set to get worse at the same time as demand is becoming greater.
It said research from the London School of Economics (LSE) found that around half a million older and disabled people who would have got care in 2009 are already no longer receiving it, while cuts in social care will amount to an estimated £4.3 billion by the end of the decade, according to the Local Government Association (LGA) and Association of Directors of Adult Social Services (ADASS).
But the CSA said it welcomed certain aspects of the act, such as its promotion of well-being and integration, giving older and disabled people greater control over their lives, new advocacy rights and new rights for carers.
CSA chairman Richard Hawkes said: "The Care Act is a bold and ambitious piece of legislation. But it will only live up to its promise of a genuinely preventative system that promotes well-being if it is properly funded.
"Chronic underfunding of social care has seen dramatic year-on-year rationing of support for older and disabled people and their carers, excluding hundreds of thousands of people from the support they desperately need.
"Equally, while we welcome a national threshold for eligibility, by setting the bar at such a high level, the Government has ensured that the year-on-year rationing that has seen people squeezed out of the system will continue.
"Ultimately, social care is an election issue and whoever forms the next government needs to urgently address the crisis in care funding, as well as in the health system.
"Anything else is simply a false economy and the reforms being implemented from today are built on sand and unable to live up to their promises."
Janet Morrison, chief executive of older people's charity Independent Age, said: "The Care Act has the potential to radically improve the lives of older people but could fall at the first hurdle for lack of funds.
"Thousands of frail and elderly people don't get any help at all at the moment with basic tasks such as washing, dressing and eating. Without proper funding to plug the black hole in social care funding - estimated by councils to be £4.3 billion by the end of the decade - this problem looks set to get worse despite the bold and welcome ambitions of the act."
Richard Kramer, deputy chief executive at Sense, said: "We are pleased that the Department of Health has listened to the concerns of people who are deafblind and incorporated a number of positive changes into the Care Act.
"For example, the guidance on how to ensure deafblind people's needs are properly assessed has been strengthened and much improved.
"However, we would urge whoever forms the next government to commit to properly investing in social care.
"Without more funding the Care Act will fail to meet its objectives and many disabled and older people will continue to fall victim to the rationing of care, leaving them at risk of missing out on the vital support they need to lead active lives and be part of the community."
George McNamara, head of policy and public affairs at Alzheimer's Society, said: " Our social care system is on the brink of crisis.
"All political parties must commit to funding the social care black hole or risk letting down the most vulnerable in society - they must not become scapegoats by politicians in the pursuit of populist votes. Failure to do so will only result in higher costs to the NHS.
"While the reforms in the Care Act are a step in the right direction, unless properly funded they will still not help the vast majority of people and do nothing to alleviate the enormous burden on unpaid carers."