The public sector auditor has warned of shortcomings in the Government's social care reforms, saying many users are struggling to cope with the new system.
Under the "personal budgets" scheme, local authorities give cash direct to users to choose their own care services, rather than having the council decide for them.
But a report by the National Audit Office (NAO) indicated some were finding it difficult to purchase care, with as few as half saying their local authority had made it easy for them to get service information and advice.
A growing number of users are employing personal assistants to help them with their care, but almost a third (31%) are finding it difficult to cope with being an employer, the research found. And although local authorities are responsible for those who fund their own care if they run out of money, the majority (60%) of councils do not know how many "self-funders" there are in their area.
Few councils were found to offer formal support to help prevent people falling back on state funding and the NAO estimated the total cost to the taxpayer of the state having to pay for self-funders who run out of money could rise from Â£500 million to Â£1 billion per year by 2035.
Examples of good practice in implementing personal budgets were identified in various local authorities, but the report - Oversight of User Choice and Provider Competition in Care Markets - recommended that these be more widely shared as they were currently very localised.
Amyas Morse, head of the NAO, said: "As the population ages and more pressure is put on social care, the Department of Health must ensure that its oversight of the care market is robust, that people have access to the information and support that they need and that it has arrangements in place in the event of large providers getting into financial difficulty."
Labour MP Margaret Hodge, chair of the Committee of Public Accounts, said encouraging user choice and provider competition by extending the use of personal budgets was a welcome aim. But she added: "The failure of Southern Cross is a stark example of why the Government needs plans in place for when large providers of social care get into difficulties. Individuals need support in managing their budgets and local authorities must manage the costs to the taxpayer."
The personal budgets scheme was started by the last Labour government and championed by Ivan Lewis, the former care services minister.
Care services minister Paul Burstow said: "The National Audit Office report is a helpful contribution to this work on how local authorities can achieve value for money so that more users and carers - a large proportion of whom are not receiving state support - are able to exercise choice in local markets. The report's comments on provider failure are a helpful contribution to our wider work on market oversight for social care. As outlined in a written ministerial statement last week, the Government is considering whether additional measures of market oversight are required. We will shortly be publishing a discussion paper seeking views on this issue."