Probation staff could take strike action over pans to privatise the supervision of offenders.
The National Association of Probation Officers (Napo) is to ballot its members on industrial action over a controversial sell-off of parts of the service.
Fears for public safety have been raised over proposals for private companies to supervise offenders classed as low-medium risk.
The probation service would continue to monitor high-risk offenders under the plan, which would take effect by October 2014.
Napo claims private firms would provide an inferior service, and has pointed out that the majority of serious further offences are committed by people classed as low-medium risk.
The union has accused the Ministry of Justice of failing to give proper assurance over the effect on public safety and not consulting properly over changes to staff terms and conditions.
Napo members have only taken strike action twice in the union’s 101-year history.
Ian Lawrence, Napo’s general secretary, said: “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.
“The government showing that it does not care about the jobs and livelihoods of my members and their high-quality professional skills is bad enough, but not caring about community safety is a downright disgrace.”
But Justice Minister Jeremy Wright said the changes would improve the probation service.
He said: “More than 600,000 offences were committed last year by those who had broken the law before, despite spending £4bn 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 forwards.
“The National Offender Management Service has well established contingency arrangements to deal with any potential industrial action.”