Prison staff in Swansea and around the country have begun protest action over "unprecedented" violence and safety concerns in prisons.
Members of the POA, the trade union for prison staff, have been asked to demonstrate outside prisons in England and Wales from 7am "until instructed otherwise".
It follows a damning report warning of a "dangerous lack of control" at HMP Bedford, their union said.
The move has been described as "unlawful" by the Government. Prisons minister Rory Stewart said: "Prison officers do vital and important work and we urge them to return to their duty stations, in line with their obligations to the law and the prison service.
"It's irresponsible for the POA to encourage their members to take this unlawful action. We are deploying our contingency plans but, by not turning up for work, these prison officers are putting their fellow staff and inmates at risk."
The Ministry of Justice (MoJ) said it would be seeking an injunction to stop the action.
POA General Secretary Steve Gillan said the majority of prison officers would be outside prisons in car parks in protest against the "unprecedented decline in health and safety standards over past six years".
The POA, which said it has 20,000 members in England and Wales, said about 90% of prison staff were members of the union.
On Thursday, Chief Inspector of Prisons Peter Clarke raised the alarm over the potential for a "complete breakdown" in order and discipline at HMP Bedford.
Official figures published in July revealed that assault and self-harm incidents were continuing to rise, both reaching new record highs.
In the 12 months to March, there were 13,119 incidents of finding drugs in prisons - a rise of nearly a quarter (23%) compared to the previous year. In particular, new psychoactive substances such as Spice have been described as a "game-changer" for safety behind bars.
Discoveries of mobile phones and SIM cards are also on the rise, going up by 15% and 13% respectively in 2017/18.
Overcrowding remains a key issue, with the prison population almost doubling between 1993 and 2016.
The prison population is forecast to "steadily" rise by more than 3,000 over the next five years, according to Ministry of Justice analysis, reaching roughly 86,400 places in March 2023.
The MoJ said it doubled the prison sentence for anyone who assaults prison officers on Thursday.
It is also investing £40 million to improve the estate and tackle the drugs problem, with 3,500 new officers to help ease the burden.