The Government "does not fully understand" the scale of the problems faced by local councils and care providers in looking after increasing numbers of elderly and disabled people despite funding cuts, a public spending watchdog said.
Reforms are "risky, are not supported by new money, and do not acknowledge the scale of the problem", the Commons public accounts select committee said - calling for a more "realistic timetable" for implementation.
The MPs called for better co-ordination between Whitehall departments - for example on the impact of benefit cut and on the increase in welfare spending and reduction in tax revenues caused by millions being forced to give up work to look after relatives.
And they expressed serious concerns that the financial squeeze had led to at least 220,000 care workers' pay being driven below the minimum wage, risking a decline in standards.
Social services chiefs have warned that the Government has significantly underestimated the extra costs involved in reforms such as imposing a minimum eligibility threshold at which local authorities have a duty to provide support.
Cash-strapped councils - which face further funding cuts - had restricted help to only those in the greatest need in the face of an 8% real-terms cut in funding.
The committee said the Department of Health and the Department for Communities and Local Government " do not know whether the care system can become more efficient and spend less while continuing to absorb the increasing need for care".
"We are concerned that the departments have not fully addressed the long-term sustainability of the adult social care system and that its policies to drive change (the Care Act and the Better Care Fund) are risky, are not supported with new money and do not acknowledge the scale of the problem."
"New duties are being introduced as local authority budgets become increasingly constrained with many local authorities already cutting their social care budgets.
"The departments neither understand the scale of some of these challenges nor how much it will cost to implement the changes."
The report concluded: "I t may not be feasible for local authorities to implement all the proposed changes to the intended timetable.
"The departments should quantify the new burdens the Care Act will introduce for local authorities, establish a realistic timetable given the financial constraints, and acknowledge the limits on the sector's capacity to absorb the growing need for care with falling public funding."
Government claims that a wide variety between costs in individual areas showed some councils could make efficiencies were undermined by National Audit Office research suggesting differences were mostly down to social, economic and demographic factors, the report said.
The MPs said they were "astonished" at evidence that up to 220,000 care workers earn less than minimum wage "and seemingly little has been done to rectify this" as well as around one third being on zero-hours contracts.
"Local authorities' cost savings have been achieved by paying lower fees to providers, which has led to very low pay for the care workforce, low skill levels within the workforce, and inevitably poorer levels of service to users," they said.
Doubts were raised in the report about the practicality of a plan to make GPs formally accountable for co-ordinating an older person's care across multiple professions and the capability of the Care Quality Commission to handle increased monitoring.
Committee chair Margaret Hodge said: "Whilst we welcome the commitment to this agenda of the departments involved, we are concerned that they do not fully understand the scale of the challenges facing local authorities, or the costs associated with implementing the Care Act.
"The act introduces new burdens on local authorities and requires unprecedented levels of coordinated working between central and local government and across local authorities and health bodies.
"The departments recognise the complexities and risks involved but we are not convinced that the responsible bodies will deliver on these ambitions and are concerned that they are raising expectations too high."
On the issue of pay she said: "In some areas, whilst local authorities might pay private providers £13 an hour, the worker only earns the minimum wage of around £6 per hour. It is also unacceptable that around one third of the workforce are on zero-hours contracts.
"At the same time 2.2 million people have had to give up work to care for family members, at extra cost to the Government through the benefits bill."
Janet Morrison, chief executive of charity Independent Age, said: "The Public Accounts Committee report should act as a stark warning to the Government that its flagship Care Act might fail for lack of money.
"Providers of services and representative groups have been saying this for months. It is simply not good enough for the Government to just trust to luck that local authorities will find the efficiency savings to fund a transformation of social care. All the evidence shows that councils are failing even to keep up with current demand.
"The PAC is right to highlight the fact that one of the unfair consequences of current underfunding is low pay for care workers. The committee raises the disturbing possibility that this may be linked to an increase in safeguarding reports.
"While we believe that poor working conditions can never be used to excuse mistreatment, we agree that there is an urgent need for more research into the extent and causes of mistreatment, poor care and abuse."
A Department of Health spokesman said: "The Care Act will make the social care system fairer.
"We know we need a skilled, valued and fairly paid workforce and are working with councils to clamp down on rushed visits and ensure they work with providers that have fair pay, terms and conditions for staff.
"The Better Care fund will help the NHS and local authorities work better together to reduce admissions and help people live more independently."
Richard Hawkes, chair of the Care and Support Alliance umbrella group representing 75 charities and organisations, said: "This is the third warning in a week that the care system is on its knees.
"Every day our 75 organisations hear horror stories of older and disabled people who struggle to get the support they need to simply get up, get dressed and get out of the house.
"This is putting unbearable pressure on family carers - as well as older and disabled people themselves.
"The new Care Act is a bold and ambitious bid to address the crisis - it will end the postcode lottery, ensure carers get more support and promote wellbeing and personalised support.
"At the same time Better Care Fund plans to integrate health and social care move us closer towards a preventive system that keeps people out of hospital and out of crisis care.
"But sitting behind this is a bigger picture of chronic underfunding which has led to a dramatic year-on-year rationing of care.
"The Care Act with its emphasis on prevention is a vital part of the solution, but the Government must act now to put the social care system on a sustainable financial footing."