The Association of Directors of Children's Services (ADCS) has said that while it welcomes government efforts to improve the quality and confidence of the children and family social work workforce it has concerns about the way the accreditation system will be introduced.
Under current plans, the system will not be fully introduced until 2019, with the arrangements initially being introduced voluntarily at 31 councils first.
An ADCS response to a consultation on the plans states that voluntary implementation of the system "poses a number of risks which will further destabilise an already fragile workforce".
A chief concern raised by the ADCS around the National Assessment and Accreditation System (NAAS) is that it will not be fully funded, leaving councils to cover the bulk of training costs for social workers taking part.
This would hit struggling councils in areas with high levels of deprivation particularly hard as they "are unlikely to have the resources or capacity to participate in a voluntary scheme".
The ADCS fears that this could result in a "tiered social work profession", based on those who are accredited and those who are not.
This could lead to social workers' professional judgment being unfairly questioned on the basis of whether they are accredited or not, the response adds.
Rachael Wardell, ADCS workforce development policy committee chair said: "If the government implements assessment and accreditation for the approved child and family practitioner and the practice supervisor statuses, this must be mandatory, rolled out at pace and fully funded by government as a new burden.
"Without mandation, it is unlikely that the NAAS will be a nationally consistent mechanism."
The ADCS response also calls into question the £23m cost of setting up the system and says this could be better spent on supporting frontline and early intervention services adding that "this initiative does not provide good value for money".
In January the government announced details about the system, which is intended to boost the quality of social work. A decision on whether it should be mandatory is expected to be made in 2019.
Through the scheme there are three different statuses of accreditation: child and family practitioner, practice supervisor and practice leader.
The Department for Education has been contacted for comment.