Probation officers are close to striking over Government plans to privatise the service after voting overwhelmingly in favour of industrial action.
Announcing the results of a ballot at its AGM in Llandudno, the National Association of Probation Officers (Napo) said 84.4% of members supported strike action on a 46% turnout. A date for the strike is yet to be fixed.
The union previously registered a trade dispute over Justice Secretary Chris Grayling's proposals to transfer most of the service to private firms such as G4S and Serco.
Ian Lawrence, Napo's general secretary, said: " We now have a mandate for industrial action that we shall be pursuing with vigour but as always Napo will be seeking to avoid this if possible by way of further negotiations with ministers."
If a strike goes ahead, it will be only the fourth time in its 101-year history that Napo will have taken such action.
Mr Lawrence said: "Napo does not take strike action lightly, but we strongly believe that decimating the award-winning public sector Probation Service and selling it off to the likes of G4S and Serco will result in increased re-offending rates, a lack of continuity in risk management, and will see the privateers making huge profits at the expense of victims, offenders and taxpayers.
"We want to raise public awareness of what these proposals will mean to the communities and put a halt to Grayling's plans until there has been a full review of his plans and a proper parliamentary debate."
Napo previously claimed negotiations with the Ministry of Justice over its Transforming Rehabilitation reforms had been ''seriously compromised'' as a result of the department's ''interference'' in the consultation on the proposals.
A package of £450 million-worth of contracts has been offered 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.
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 is being conducted of all contracts held by Serco and G4S, two of the country's biggest private providers of public services .
The audit, triggered by disclosures 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 Mr Grayling intended to allow Serco and G4S to bid for the probation service - though the firms will not be awarded anything until the Government's audit is completed.
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 about how it operates.
Justice Minister Jeremy Wright said: "This is a strike against giving supervision and support to 50,000 short sentenced offenders each year, who are currently released without any supervision and go on to commit so much crime in our communities.
"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 have well established contingency arrangements to deal with any potential action. We will continue to support staff and engage with unions as our reforms move forwards."
Max Chambers, head of crime and justice at think-tank Policy Exchange, said: " It's no surprise that some probation staff are worried about the future, but striking is not a very clever move and could put the public at risk.
"The time is fast approaching when the Probation Service will need to get on board with these reforms.
"There are huge opportunities that payment-by-results can offer staff, including greater professional discretion, more freedom to innovate and the chance to have a stake in their own mutual organisations."