Kevin Curley, chief executive of local umbrella body Navca, has written to third sector minister Angela Smith setting out his fears that the new vetting and barring scheme will deter people from volunteering and will be expensive and time-consuming for small voluntary and community groups.
The letter warns that the scheme, which was announced in September 2009 and requires volunteers and staff working with children and vulnerable adults to register with the Independent Safeguarding Authority, could "curtail the spontaneity and informality that characterises voluntary and community groups".
It adds: "Many volunteers will regard the checks as an intrusion into their privacy and be concerned that their past activities will be made public."
It warns that some people could be dissuaded from volunteering, especially if they have only fleeting contact with beneficiaries and cannot understand why they are covered by the legislation.
The letter asks for government funding to help voluntary groups with the costs of the scheme. Although registration with the ISA is free for volunteers, Navca points out that most small organisations administer their volunteers through umbrella bodies that charge for staff time.
It also calls for guidance on how trustees, members of self-help groups and people with fleeting but regular contact with beneficiaries will be affected under the scheme.
A spokeswoman for the Cabinet Office said:"Although we have not received the letter, we will look into any concerns raised. We work closely with other government departments and third sector organisations, and we are working on guidance for volunteering organisations."
A Panorama programme on child protection looks at the vetting and barring scheme on BBC1 this evening.