Councils spend significantly more on adult services they provide than on care they commission, new figures reveal.
Statistics released last week by the Health and Social Care Information Centre show that unit costs for services for older people, disabled people and those with learning difficulties or mental health problems are significantly higher for in-house services.
The figures have prompted concerns over the quality of care in the independent sector, where the majority of social care is now provided.
In terms of residential care, older people's services that are outsourced cost £361 per person per week in 2004-5 - only 59 per cent of the cost of ones run by the council.
For mental health services, the unit cost for commissioned services was 70 per cent of council services' cost, for learning difficulties it was 71 per cent and for disability services it was 78 per cent.
For home care, the average hourly rate for external providers was 65 per cent of the tariff for council services.
Anne Williams, co-chair of the Association of Directors of Social Services' resources committee, said the figures in part reflected the lower wage costs in the independent sector, and said this raised questions about the quality of care.
She said: "The current financial climate means that people who work in social care, most of whom are in the independent sector, are poorly paid for the responsibilities they take on.
"That is a concern in terms of how you provide the kind of quality service that people want."
Margie Jaffe, national officer at Unison, said independent sector providers rarely offered pensions or sufficient training, adding: "Outsourcing inevitably leads to lower quality standards. We see money saved but at the cost of services."
However, Martin Green, chief executive of the English Community Care Association, said the figures showed the greater efficiency of the independent sector.
He said: "Local authorities should be reminded that providing in-house reduces the numbers of people that they can offer support to."
He also claimed the figures showed that independent sector providers were not being passed the full costs of care, and reiterated the association's call for independent assessments of care costs in every council area.