Prison officers staging pension protest meetings have been told they face the threat of a high court injunction because they are banned from taking industrial action.

The Prison Officers Association (POA) says the meetings have been taking place at "80% of prisons" since staff came to work at 7am on Thursday and will continue until they are called off by the union's national executive.

Steve Gillan, the POA general secretary, told the Guardianit had been warned by the Treasury solicitor that ministers consider the protest meetings amount to industrial action and will seek an injunction if they continue.

The prisoners at the jails affected have been put on a "lockdown" regime. Gillan said minimum cover arrangements were in place to ensure prisoner safety.

The POA is unhappy about plans to link the normal pension age for prison officers to the state retirement age. "The state pension age will ultimately rise to 68 and it is unrealistic to expect 68-year-olds to walk landings and grapple with prisoners aged 20 or 21," said Gillan. "Unfortunately our case has fallen on deaf ears and prison officers have no other option but to protest to gain public attention."

The Ministry of Justice confirmed that prison officers could face an injunction. "Ministers are keeping all options for bringing this action to an end under consideration," it said a statement.

Michael Spurr, the chief executive of the National Offender Management Service, said: "I am extremely disappointed that the POA has taken this unlawful action. We have implemented our contingency plans, and our priority is to protect the public and ensure that prisons remain safe and secure.

"In 2007, the POA agreed that the normal pension age for new prison officers would be 65, in line with all other civil servants. The government has been in constructive discussions with the POA about further pension reform and it is deeply regrettable that this action has been taken now."