By Nick Triggle
Health reporter, BBC News
Social care is means-tested in England
The government has "significantly underestimated" the cost of delivering free social care, council chiefs say.
A poll of 61 directors of social services suggested providing care to those most in need costs £200 a week - double the government's estimate.
The Association of Directors of Adult Social Services (ADASS) said it backed the idea - planned for England - but it wants a funding rethink.
The government said it would consider the findings.
Free personal care is likely to be one of the key planks of Labour's election strategy.
'Most in need'
The move was announced by Gordon Brown at the party's annual conference and a bill included in the Queen's Speech set out the plans.
Some have criticised the government for rushing to push through legislation before waiting for the consultation results on the proposals.
But Health Secretary Andy Burnham said it was time to make the system fairer.
The service - for people needing help eating, washing and dressing - is currently means-tested.
But ministers want to see English councils provide it free to an estimated 280,000 people, mostly the elderly although some have disabilities, to mirror the package available in Scotland.
They estimate that people who fall into the most critical bracket need six and a half hours of care, but the evidence provided by the survey suggests it is double that.
It means councils would have to find more than £500m a year - on top of the £420m provided by government - to fund the scheme if it comes into place in the autumn as forecast.
ADASS president Jenny Owen said the "pressures on local government will clearly be well above the sum estimated".
She added: "We are keen to meet with the department to help clarify and resolve these outstanding issues."
A Department of Health spokesman said: "We are in the process of consulting on aspects of the bill and will feed ADASS's new contribution into this."
The concerns raised by social care chiefs come after charities said the plans may lead to councils restricting access for people who do not qualify for free care.
The plan covers only those with the most critical needs, often people who need help in the mornings, lunchtime and evenings.
But this accounts for just about half of the total people receiving home care.