Prison officers are to hold an "indicative ballot" on whether they should take industrial action, including strikes, to try to halt the government's decision to privatise more jails – a move that could lead them to breaking the law.

The justice secretary, Kenneth Clarke, has already revealed that troops have been put on standby to step in if warders stop work. His announcement last month that Birmingham prison would be the first existing prison to be privatised prompted the threats to disrupt the prison service.

There are already 11 privately-run prisons but all were newly built. The security firm G4Swill run Birmingham from October and Featherstone 2, a new "supersize" jail with 1,600 places in Featherstone, Yorkshire.

The executive of the Prison Officers' Association, decided on the ballot, which will involve about 30,000 prison officers in England and Wales, on Wednesday.

Colin Moses, its chairman, 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."

Steve Gillan, the association's general secretary, said: "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."

Although the association had a mandate to recommend industrial action, that was now two years old, said Gillan. "We are instituting an indicative ballot to re-engage with our members to make sure that is what they want to do. We believed it was the best way forward so we could get an up-to-date view."

The result could go either way. "I am happy with either outcome, to be honest," he said. The union would consider the legality of anything it did once the indicative results were in.

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 Ministry of Defence and police to ensure that our contingency arrangements are as strong as possible."

Clarke said last month the army had not been sent into jails "within living memory".

The decision to train up to 3,000 soldiers in control and restraint techniques and in running prisons was made after ministers studied what happened when the POA went on strike in August 2007. It was 12 hours before a high court injunction halted the action. It happened while Labour had suspended the legal ban on industrial action by prison officers. A reserve power reimposing the ban was introduced in 2008.

Troops would be unlikely to be able to cover a national strike but there might be enough to run a handful of prisons shut down by the strongest POA branches.