Probation officers are supervising up to 80 offenders at any one time - leaving the public at greater risk, a watchdog has warned.
HM Chief Inspector of Probation Dame Glenys Stacey highlighted large caseloads in a report on services in Staffordshire and Stoke.
The inspection is the latest to examine arrangements following a controversial shake-up of the regime for managing offenders in the community rolled out in 2014.
Under the overhaul, probation services in England and Wales were divided into a new National Probation Service (NPS) and 21 privately owned Community Rehabilitation Companies (CRCs).
High-risk individuals are managed by the NPS while CRCs are responsible for other offenders.
Dame Glenys said the two organisations providing probation services in Staffordshire and Stoke need to improve the quality of their work.
She said: "The CRC is making progress, and its ambitious plans for an effective probation service, to help turn people's lives around and reduce reoffending, are delivering some good results in places.
"But too much of its bread and butter work to protect the public was wanting."
The report found some officers were responsible for up to 80 cases.
Dame Glenys said: "High individual caseloads are becoming commonplace in CRCs.
"Of course CRCs must manage within anticipated resource, but the public is at greater risk when officers are spread too thinly and if quality assurance is not robust."
She added that the work of the NPS in the region was of "sufficient quality" but there were "notable weaknesses" in places.
The Staffordshire & West Midlands CRC is owned by the Reducing Reoffending Partnership (RRP).
Catherine Holland, chief executive of the RRP, said: "Our team of probation experts work hard to keep the people of Staffordshire and Stoke safe by reducing reoffending.
"We welcome this inspection report, which identifies many areas of good practice, and will use its findings to further strengthen our work."
A Ministry of Justice spokeswoman said: "We hold providers rigorously to account for their performance.
"A robust action plan for Staffordshire and Stoke CRC is in place to address the concerns raised by the inspectorate, including developing a new assessment tool to look at offenders' areas of risk. We will continue to monitor their progress closely."
A review of the probation system in England and Wales is being carried out, with findings due in April.