Thousands of prison officers are to be balloted on industrial action, including strikes, in protest at the Government's privatisation plans for Birmingham and Featherstone jails.
The move by the Prison Officers Association (POA) sets the union on a collision course with the Government because it is unlawful for warders to take industrial action.
Around 30,000 POA members in England and Wales will take part in an indicative ballot, which will be considered by union leaders before they decide their next move.
POA general secretary Steve Gillan said that prison officers were "angry" at the Government's privatisation plans for Birmingham and Featherstone.
Justice Secretary Kenneth Clarke revealed last month that troops had been put on stand-by as the Prison Service braced itself for industrial disruption.
The POA reacted angrily to news that Birmingham prison, in Winson Green, and the new Featherstone 2, in Wolverhampton, are to be run by private security company G4S.
He told MPs: "If people are so unwise as to take industrial action in prisons, the situation can rapidly become far worse than in a normal strike because we start getting disorder among the prisoners."
The POA said last month that it would not make a "knee-jerk" reaction, but has now decided to hold a ballot of members.
Mr Gillan said there would be no recommendation from the POA leaders, adding: "We are just seeking our members' views. The anger is still there and some of our members are now working to contract to show their feelings.
"The ballot result will determine what we do in the future."
A Ministry of Justice spokesman said: "Our aim is to avoid any form of industrial action and we will continue to talk to all unions to do everything possible to achieve that.
"However, given the risks and complexities involved in running prisons, it is sensible and appropriate to review our contingency plans for dealing with widespread industrial action.
"There was a previous trilateral agreement with the police and the military for the provision of support to prisons and we are currently reviewing those arrangements and working with the MoD and police to ensure that our contingency arrangements are as strong as possible."
POA chairman Colin Moses said: "It is vital that the members have their say through the ballot box and decide what course of action they are prepared to take to support their colleagues, who have been sold off to the private sector.
"Prisons should not be run for profit and whilst Ken Clarke took great pride in announcing the privatisation of these prisons, the POA remains of the view that it is the state's responsibility to imprison its citizens and not profiteers."
Mr Gillan added: "It is important to gauge the view of our members through the ballot box, and if it is their will the POA will take clear and decisive action against the privatisation of Birmingham and Featherstone 2.
"We will continue to hold weekly lunchtime protest meetings and persuade our members to follow the policies of the union."