The justice secretary, Kenneth Clarke, has confirmed that he is willing to call in the army should "serious trouble" erupt over his decision that the private security company, G4S, should run Birmingham prison – making it the first public sector jail to be privatised.

"We have to be prepared should anything go wrong," he told MPs.

The Prison Officers' Association (POA) said it would not rule out industrial action in protest at the decision, despite the fact that such action may be illegal.

Clarke said the military contingency plans, involving up to 3,000 trained regular soldiers, were needed because industrial action involving prisons "can rapidly become far worse than an ordinary strike".

The justice secretary told MPs: "Nobody is contemplating a military takeover of a prison; the governor will still be in charge. But it is only proper that we have made military preparations that would be required if serious trouble erupts."