Up to 1,000 probation officers are expected to appeal against jobs assigned to them under a shake-up of the rehabilitation service in England and Wales.
A total of 119 of 553 appeals have been successful so far, the National Association of Probation Officers (Napo) said.
But the total number of attempted appeals is expected to rise to around 1,000, the union added.
A package of £450 million-worth of contracts has been offered to private firms and charities , covering the supervision of 225,000 low and medium-risk offenders each year on a payment-by-results basis.
Contracts are to be split across private community rehabilitation companies (CRCs) in 20 English regions and one Welsh region, while the National Probation Service (NPS), a new public sector organisation, will be formed to deal with the rehabilitation of 31,000 high-risk offenders each year.
Ian Lawrence, general secretary of Napo, said: "We are seeing a high number of appeals and anticipate there being about 1,000 in total once all the trusts have finished the staff assignment process.
"However, the number of grievances - expected to reach 2,000 - across probation trusts better reflects the feelings of our members.
"Staff have been left anxious about their future employment with still many unanswered questions.
"The Transforming Rehabilitation agenda has been flawed from the outset and it's our members that are currently bearing the brunt of this and that has led to staff leaving the service or registering grievances."
The figures, first revealed by the B BC, show the appeals cover 18 probation trusts, out of 35 overall, from which Napo has so far obtained figures.
Thousands of Napo members went on strike last November over the proposals.
More than 700 organisations from across the world have expressed interest in the contracts, including hundreds of British firms.
A Ministry of Justice spokeswoman said: " As of February 4, only 3% of probation staff have appealed against their new roles, and only a fifth of those appeals have been upheld.
"Staff whose appeal was upheld are in the process of being reassigned posts in the NPS or CRC as appropriate."