The National Association of Probation Officers (Napo) says contractor Working Links has told it 40% of jobs in the counties will go.
Working Links won the contract for the counties in the part-privatisation of the probation service in 2014 .
The firm said it was still in consultation but it expected "fewer jobs being lost than those stated".
Read more on this story as it develops throughout the day on our Local Live pages
Mike McLelland of Napo said a 40% cut meant a total of 117 jobs being lost.
"Napo is concerned these staffing cuts will result in far less face-to-face contact with offenders," he said.
"The consequence of this will be to seriously compromise the ability of probation providers to both assess and manage risks of serious harm and reoffending."
Working Links said: "We are still in consultation but latest projections show fewer jobs being lost than those stated.
"Our front line delivery will not be negatively impacted by our proposed changes. At all times we will ensure we deliver a safe service.
"We are committed to reducing reoffending and protecting the communities in which we work."
A Ministry of Justice spokesperson said: "Public protection is our priority and we continue to work closely with probation providers to maintain appropriate staffing levels and to rehabilitate offenders effectively."
More than 100 probation jobs are being cut in Devon, Cornwall and Dorset, a union has claimed.