Probation officers in England and Wales are to be balloted for possible strike action over Government plans to privatise the service.
Ian Lawrence, general secretary of the National Association of Probation Officers (Napo) union, is to formally register a trade dispute over the proposals, which will see the vast majority of the service sold to private firms such as G4S and Serco.
Napo members, who are employed by the Probation Service in England and Wales, are then to be balloted for industrial action including striking, the union said.
The union claims negotiations with the Ministry of Justice over its Transforming Rehabilitation reforms have been "seriously compromised" as a result of the department's "interference" in the consultation on the proposals.
Mr Lawrence said: " Our members along with other stakeholders have opposed these plans from the start because we know that by fragmenting the Probation Service and selling it off to the likes of G4S and Serco it will lead to a poorer quality of service and undermine risk management which in turn will put communities at risk."
Napo members have only been on strike twice in the union's 101-year history.
Mr Lawrence added: "It's not something our members take lightly but they feel that these proposals will have such an impact on public safety that they have no choice but to take a firm stand on this issue on behalf of the communities they serve."
A package of £450 million-worth of contracts was last week offered up to private and voluntary sector organisations, 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 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.
Mr Lawrence added: "The very fact that Government doesn't trust the private and third sector to manage high risk of harm offenders speaks volumes."
More than 700 organisations from across the world have expressed interest in the contracts, the MoJ said, including hundreds of British firms.
A Government-wide review of all contracts held by Serco and G4S, two of the country's biggest private providers of public services, is currently being conducted.
The audit, triggered by revelations that both firms had overcharged the Government for criminal-tagging contracts, prompted calls for the Ministry of Justice to abandon its plans to privatise the probation and prison service.
But it emerged that Justice Secretary Chris Grayling does intend to allow Serco and G4S to bid for the probation service contracts.
Napo has called for the proposals to be tested and claims recent reports from America, where some states have already outsourced their Probation Service, suggest there are concerns in how it operates.
Mr Lawrence said: "If the Government is confident that their plans will work then they should run a pilot that is properly evaluated and publish its findings to parliamentary and public scrutiny.
"There is too much at stake here to simply roll out a wholesale sell-off of the Probation Service and the public have a right to know how these changes will impact on them."
G4S and Serco will not be awarded new contracts until the Government's audit is completed.
Justice Minister Jeremy Wright said: " More than 600,000 offences were committed last year by those who had broken the law before, despite spending £4 billion a year on prisons and probation.
"The public deserves better and we are committed to introducing our important reforms, which were widely consulted on. We will continue to support staff and engage with unions as our reforms move forward.
"The National Offender Management Service has well-established contingency arrangements to deal with any potential industrial action."