A fresh wave of industrial action will be held in jails in England and Wales in a dispute over pay and pensions, the prison officers' union has said.
Prison Officers Association members will withdraw from voluntary duties, including manning "Tornado" teams which respond to outbreaks of disorder.
An overtime ban will also be phased in from April.
The government said such action was "unlawful" and it would seek an injunction to stop it happening.
From Wednesday, staff are being instructed to withdraw from a range of voluntary roles, which also include working as a first aider or hostage negotiator.
However, the POA said its members would respond if lives were at risk.
The BBC's home affairs correspondent Danny Shaw said the planned overtime ban could cause serious problems in jails that are short of staff and struggling to fill vacancies.
In a briefing paper seen by the BBC, the union said: "The POA condemns the systematic failure of Noms (National Offender Management Service) to provide safe, decent and secure prisons, failures which have created a prison service in crisis.
"More and more members are being assaulted every day, the increase in self-inflicted deaths and daily security breaches are unacceptable and as a result of staff shortfalls and budget cuts."
A spokesman for the Ministry of Justice said it had made a "good offer" to the union in December, which was endorsed by its leaders, but not members.
"We are working hard to retain the invaluable experience within our workforce and want to recognise the expertise and dedication of prison staff," he said.
Last week, ministers said pay rises of up to £5,000 would be given to some officers at 31 prisons in London and south-east England with recruitment problems.
But the POA said the pay rise had "incensed" its members, many of whom would not benefit.
In November, thousands of prison officers staged a 24-hour strike because of health and safety concerns.
The High Court ordered the POA to suspend the protest and talks later took place with the government, focusing on pay and pensions.
The Ministry of Justice announced last month that the National Offender Management Service would be scrapped and replaced with a new prison and probation service aimed at cutting crime and reforming offenders.