Questions are being asked about what impact a flagship government scheme to improve the care of vulnerable patients in England will have.
The £5.3bn Better Care Fund will be launched in April 2015 to encourage greater integration between the NHS and social care.
But a review by the National Audit Office found the scale of potential savings was overstated to start with.
The government said it disagreed with the criticisms.
However, the NAO said even now, the revised plans contained "bold assumptions".
Last month, ministers championed the impact the fund was going to have after announcing nearly all 151 local plans had been signed off.
The details of the schemes vary, but all involve some form of joint working between social workers, nurses and other community staff to improve the support offered, particularly to elderly people.
Where has the money come from?
The £5.3bn Better Care Fund is not new money as such. Instead it draws on existing funding streams that are being ring-fenced for this purpose.
The pot was originally set at £3.8bn. That included £3.3bn from the core NHS budget with the rest from money set aside for carers and capital expenditure.
This has been topped up by extra funds that have been put in by local areas that wanted to create bigger pooled budgets.
The £5.3bn represents less than 5% of the combined NHS and social care budgets.
The aim is to keep patients out of hospital and - where possible - living independently in their own homes.
Originally it was hoped the measures would save the NHS £1bn in 2015-16.
But when the local plans were initially submitted earlier this year, £55m of savings was identified.
More work on the plans was ordered and the figure has now been increased to just over £300m, although this has meant there is less time to prepare for implementation. Most of these savings come from a forecast reduction in emergency admissions of 3.1%.
But the NAO still said the plans were "over-optimistic".
It said local areas needed better support to maximise what was an "innovative" approach to changing the way the care system works.
Public Accounts Committee chairwoman Margaret Hodge said the rollout of the fund had been a "shambles".
A Department of Health spokesman said it disagreed with the criticisms.
"This is the most ambitious plan to transform care ever undertaken and we ensured detailed work took place a year ahead of the launch to allow us time to iron out the issues."
Councillor Izzi Seccombe, of the Local Government Association, added it was "too soon" to make judgements about the scheme.
But Richard Humphries, of the King's Fund think tank, said the fund was "not a substitute for the new funding needed to invest in essential changes to services".
"Given the tight timescales and absence of any new money in the fund, local areas are being expected to achieve too much, with too little, too soon."