David Hogarth, co-ordinator at Neighbourcare St John's Wood and Maida Vale, wrote to the Home Office saying he believes the CRB's decision to stop registering small organisations will deter volunteers because they will have to report to a registered umbrella body for criminal checks.
He fears charities will respond to the threat by stopping criminal checks and relying on references instead, if there is a choice.
"It is bad enough having to tell volunteers they have to go through checks without saying they must report to the offices of an umbrella body," he said. "My guess is, if we tried this, most would withdraw their application."
Hogarth's charity makes about a dozen checks a year, far below the lower limit of 200 suggested by the CRB. He said umbrella-registered bodies had no real interest in helping volunteers, whereas charities visit volunteers at home to explain the process.
Nina Savage, a volunteer service officer at St Anne's Hospice, a CRB-registered charity, said: "Volunteers might go elsewhere if we had to use a registered body because they would have to wait longer than before to hear from the CRB."
Westminster Council for Voluntary Action has taken Hogarth's concerns to the NCVO, saying that being registered allows charities to control the disclosure application process, making it less threatening. The NCVO said it was pushing for an umbrella-registered body specifically for the sector.
Adoption Matters said it would have to turn to umbrella-registered bodies as it was legally obliged to make checks on staff every three years. But it said relying on a third party would lead to delays and affect the quality of the checks.
The CRB wants to reduce the number of organisations that register with it by setting a threshold of 200 checks
An annual registration fee is planned to replace the current one-off payment
It costs £300 to become a registered body
Registered bodies have to countersign applications to confirm that applicants meet the relevant criteria and to give permission for checks to go ahead.