The NCVO is demanding that future cost savings made by Capita, the company that processes checks for the Criminal Records Bureau, be passed to charities rather than government.
Last month, the Home Office signed a revised contract with Capita. As a result, from this October it expects "a significant reduction in the price paid to Capita for processing each disclosure application".
But, according to NCVO, a statement from Home Office minister Hazel Blears implied that any efficiency savings would be kept by the Government.
NCVO has written to the Government to protest against this decision.
Chief executive Stuart Etherington said: "Future savings by Capita could offer the opportunity to considerably reduce the financial burden of criminal records checks on the CRB's customers. It would be very disappointing if this opportunity was missed.
"By passing on savings made by the CRB to its customers, the Government could provide the crucial protection that the most vulnerable in our society need, without placing a high financial burden on voluntary organisations."
Charities are still reeling from the announcement that the price of disclosures will go up by 17 per cent from April, raising the cost of a standard check to £28. That increase followed a doubling of standard checks last summer from £12 to £24.
The letter requested a meeting with ministers, at which the NCVO will attempt to clarify whether the Government plans to keep any efficiency savings. The NCVO suspects this is the likely outcome, as a Home Office spokesman said it wanted the bureau to be self-financing by 2005-6.
Pete Moorey, the NCVO's parliamentary officer, fears this deadline is too early and could result in further price rises for CRB checks.
Chris Shaw, acting chief executive of Help the Hospices, said: "Some 28 per cent of adult hospice income comes from government and yet the increases in CRB charges have already cost hospices an extra £80,000 a year."