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 third 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.
There's even more bad news piling up for Chris Grayling because Ian Lawrence also announced to delegates that a letter was in the post giving official notification of an application for Judicial Review of the whole Transforming Rehabilitation omnishambles. Add this to the news that apparently Derbyshire Probation Trust have declared that they have no intention of signing the contract variations demanded by MoJ/Noms, and there is increasing evidence to feel confident that the TR omnishambles can be defeated by lots of individual spanners being thrust into the works.
You have to hand it to Michael Spurr, Noms CEO, in having the bottle to turn up at the Napo AGM and try to defend the indefensible, and without notes. The longer he spoke and the more questions he fielded from the floor, the less convincing he sounded. In particular he had no convincing argument for the hasty replacement of Mike Maiden without any selection process.
Basically his answer was he did it because he could. It also adds further fuel to the discussion surrounding the real reason for Mr Maiden's departure. As has been noted, there was no fulsome praise for his work in being a significant architect of the new Service in the official MoJ press release, thus fuelling speculation that in reality there was a major disagreement.
Mr Spurr was described as a decent chap by Steve Gillan, General Secretary of the Prison Officers Association, and when challenged to basically say how he slept at night introducing the TR proposals, he replied that there "were certain things he would not do". To be honest, I'm intrigued to know what those are exactly in the context of TR.
Due to internet difficulties, this is being written in a very noisy branch of a well-known pub chain and I need to go to the bar. More tomorrow.