Inspectors found staff assigned to manage low and medium-risk individuals in Gloucestershire were making “heroic efforts” but faced “plainly unreasonable” caseloads.
The probation watchdog described the quality of public protection work by the community rehabilitation company (CRC) in the county as “poor”.
Although the risk of harm was assessed to an acceptable standard in the majority of cases, it was not followed through often enough or well enough, according to the report.
Dame Glenys Stacey, HM Chief Inspector of Probation, said: “The quality of assessment and planning was mixed, but in any event, plans were not being followed through anywhere near well enough and some offenders were not being seen often enough.
“As a result, the public were more at risk than necessary, and offenders who could turn their lives around were being denied the chance to do so.”
In several cases, the CRC had lost contact with an offender for “significant periods”, the report said.
In one example a man who had a conviction for assault was said to have had no contact for two months after his first officer left.
The findings are the latest in a line of critical assessments of a controversial shake-up of the system for the management of offenders in the community in England and Wales.
Known as Transforming Rehabilitation, the partial privatisation launched in 2014 saw the creation of the National Probation Service to deal with high-risk individuals, while remaining work was assigned to 21 CRCs.
Dame Glenys said: “The National Probation Service was performing reasonably well, and the public can be reassured that those people who pose a higher risk are generally being supervised to an acceptable standard in Gloucestershire, although more could be done to reduce the risk that individuals reoffend.
“The picture was much more troubling at the community rehabilitation company, where there have been drastic staff cuts to try and balance the books.
“This CRC’s work is so far below par that its owner and government need to work together urgently to improve matters, so that those under supervision and the general public receive the service they rightly expect, and the staff that remain can do the job they so wish to do.”
The Bristol, Gloucestershire, Somerset & Wiltshire CRC was supervising a total of 6,701 offenders at the end of last year.
A spokesman for the CRC said the report “highlighted many pieces of good practice as we work hard to combat re-offending”.
The spokesman said: “Overall, we note we are performing relatively well compared to other CRCs and are performing well against targets set by the Ministry of Justice.
“However, we recognise there are areas of our work which we need to improve on if we are to be successful in achieving our ambitious goals.
“We will be working even harder, alongside our colleagues in the National Probation Service, to address the issues identified and deliver the services which the public, courts and our service users require from us.”
The CRC said all of the inspectorate’s recommendations are being or have been addressed since the inspection.
Michael Spurr, chief executive of HM Prisons and Probation Service, said the CRC’s performance “does need to improve”.
He said formal action was already being taken prior to the inspection, and an improvement plan put in place to address the inspectorate’s recommendations will be closely monitored.
Shadow justice secretary Richard Burgon said: “The Tories were repeatedly warned that their reckless part-privatisation of probation would prove to be a costly disaster.
“Now we see the latest in a series of reports demonstrating that public safety is being put at risk because ex-offenders aren’t getting the support, supervision and rehabilitation they need.”