Guards fear some jobs could be lost should the government privatize the running of the prisons, The Daily Telegraph reported Friday.
Military personnel are undergoing training, the Ministry of Defense confirmed Friday. Service personnel will have 6 hours to get to their assigned prisons once a strike is declared.
They would have "non-prisoner facing duties" such as guarding the perimeter of the prison or monitoring surveillance cameras, officials said.
A spokesman for the Prison Officers' Association denied a strike was imminent, saying there was "no planned action at all."
Prison officers are forbidden by law from striking, but they have conducted wildcat strikes.
Prison officers at an estimated 80 percent of facilities in England, Scotland and Wales participated in a surprise walkout in May.