But probation officers' union Napo called for the two firms to be "disallowed" from delivering services until the audit was complete, and criticised the government's "rush to privatise the probation service".
"G4S and Serco currently hold 17 contracts with the Home Office and it is essential a completeaudit of these contracts is now conducted, " Mr Vaz said.
The electronics retailer declined to discuss any details of its continuing investigation Thursday, saying it planned a full airing of its findings once the inquiry by the board's audit committee is complete.