The letter, signed by Justin Davis Smith, chief executive of Volunteering England, and Oliver Reichardt, head of Compact Voice, which represents the voluntary sector on the Compact, voices concern that charges for the service, which is free for volunteers, would be introduced as a result of the government's review of the vetting and barring system.
"We understand the government is reconsidering the current commitment to free CRB checks for volunteers because of the wider review of the vetting and barring process," says the letter, which was also sent to Nick Hurd, the Minister for Civil Society.
It says removing the free checks would "send a strong signal to the sector that the government is not fully committed to increasing participation in volunteering".
Davis Smith told Third Sector: "We agree that the vetting and barring system should be much simpler, because the bureaucracy involved with criminal records checks can be a barrier to people volunteering. But we don't think charging volunteers to have the checks would be the right solution."
The letter says free CRB checks for volunteers are a key commitment in the Compact, the document that sets out how the government and the voluntary sector should relate to each other. It urges the government not to remove this from the Compact when a new version is published next month.
The letter adds: "Introducing a charge for volunteers would also risk undermining the good work started by the Big Society Deregulation Taskforce, headed by Lord Hodgson, by running contrary to moves by the government to reduce the bureaucratic barriers facing volunteers. We would urge you to state unambiguously that you are committed to ensuring that CRB checks remain free for volunteers."
A Home Office spokeswoman confirmed the government was reviewing the free checks as part of a review of the entire vetting and barring system.
"The terms of reference for the remodelling of the vetting and barring scheme and of the criminal records regime are currently being considered and a further announcement will be made shortly," a statement from the department added.