Adult social care in England is under increasing pressure and the government has "no idea" how long the system can cope, according to an official inquiry.
The National Audit Office also raised doubts over whether an overhaul of care services, which begins in 2015, will be as successful as ministers hope.
A lack of time and information could leave councils struggling to improve services, the report added.
Ministers say they are giving councils £1.1 billion to protect such services.
The NAO found that while demand for adult social care was increasing, spending by local authorities fell by 8% in real terms between 2010-11 and 2012-13.
Some of this was achieved by delivering care more efficiently, but researchers also found evidence that councils paid providers less, putting financial pressures on some companies who complained of being able to deliver only basic care.
The thresholds for receiving support have also increased - 87% of councils will only provide care if the need is judged to be substantial or critical.
Noting that safeguarding concerns had increased by 13% between 2010-11 and 2012-13, the report suggested that while increased awareness of abuse may explain the rise, it could also indicate a system under pressure where resources were overstretched.
The concerns expressed about the ability of councils to deliver on the requirements of the Care Bill, which MPs were debating this week, will be welcomed by the care sector.
The Local Government Association, the Association of Directors of Adult Social Services and others warned on Monday they needed £135 million in additional funding to make the necessary changes.
From next April, they will be responsible for conducting carer assessments and other measures prior to the main measures of the Bill, including a cap on some care costs, being introduced in April 2016.
The report praises government efforts to improve social care, including the creation of the Better Care Fund, which will see NHS money spent on council services.
But it criticises a lack of evidence as to how long it will take for this integrated model to deliver better services.
Initiatives from other government departments, such as changes to benefits, create further uncertainties, the NAO indicates.
Its head, Amyas Morse, said adult social care was "one of the big issues we face at present".
"There are no easy answers, but we need to think clearly and in a joined-up way about the predictable and growing challenges in years to come."
Margaret Hodge, chair of the Public Accounts Committee, said changes were being introduced when the government had "no idea" whether savings would be delivered, something that was "worrying".
"The result is unnecessary stress and an unfair financial burden on those who need care," she said.
The Department of Health said the population was ageing so "health and social care services must work differently".
A £3.8 billion Better Care Fund would link NHS and social care services and "help to keep people living independently at home, get them out of hospital more quickly and prevent them from needing more support", a spokesman added.
"We are giving councils £1.1 billion funding to help protect social care services in 2014-15."