MPs were warned yesterday of growing anxiety that government plans to merge health and social services in "care trusts" may lead to an unequal partnership, with community services squeezed to meet the acute needs of a dominant NHS.

Concerns were voiced by the Association of Directors of Social Services (ADSS), the Local Government Association (LGA), public services union Unison and charities Age Concern and Help the Aged as the committee stage of the health and social care bill began to debate the care trust clauses.

Ministers gave assurances in the summer that they did not want the NHS to swallow up social care, but these were not clearly written into the fine print of the bill. The five organisations said in a letter to Alan Milburn, health secretary, that while care trusts could be "an important addition to existing flexibilities" in terms of developing seamless services, their potential would be fully realised only if they "draw equally on the expertise and experience of both local government and the NHS".

The ADSS points out that the bill describes care trusts as NHS bodies, with the result that social services may come to be regarded as a minor partner. "There is evidence from Northern Ireland that, where health and social care has been merged, resources are invested in acute care at the expense of those in the community," the association says. There is "a danger that a medical/clinical model could begin to dominate the development of services", damaging preventative work in the community, such as the home care service.

The LGA is concerned that councils should have the right to withdraw from care trusts if they think the arrangement is not working well. The bill states that this could not be done without government permission. According to the LGA, this would mean local authorities "may find themselves delegating their responsibility for social care to the secretary of state in perpetuity".

There is also anxiety that care trusts could be imposed on areas where services were failing. Yet there are no criteria or procedures for judging failure. The five organisations are seeking "an assurance that the imposition of care trusts will genuinely be an option of last resort".

Other demands by the five include: statutory consultation with service users before any care trust is imposed; greater clarity about which services should be free and which should be charged; and protection of resources to ensure that care trusts do not cut levels of service provided by local authorities that have traditionally spent above government guidelines.

Ministers were expected to try again yesterday to persuade the social care community that it will not be taken over by the NHS. But it was understood that no government amendments were in prospect to consolidate that assurance on the face of the bill.