Scores of prison officers in the North East stopped work on Tuesday amid claims jails were “in meltdown”.
The demonstration, which began at midnight, comes after a string of high profile incidents at prisons, including an alleged murder, a riot and the escape of two inmates.
Two prisoners managed to escape from Pentonville prison, in north London, earlier this month – sparking a manhunt in which they were eventually recaptured.
Weeks earlier, an inmate died after being stabbed at the jail on October 18 in an attack which left two others injured.
And on November 6, up to 200 prisoners went on a rampage in HMP Bedford.
Terry Fullerton, North East spokesman for the Prison Officers Association (POA), said there had been an increase in attacks on staff as well as prisoner-on-prisoner assaults, and suicides had also gone up.
He added: “This is a protest action against the rise in violence and unsafe nature of jails. We just want the establishment to sit up and take notice.
“For quite some time now, we have been trying to say that the reduction in staff levels have led to the prisons being really unsafe.
“The incidents at Pentonville and Bedford are just a snapshot of what’s going on in our prisons. Our staff have had enough.”
During the protest, officers were providing emergency cover for fires and medical incidents in order to protect prisoners’ well-being, the union said.
Meanwhile, the action was branded “unlawful” by the Ministry of Justice.
An MoJ spokesman said: “There is no justification for this action.
“We have been engaged in constructive talks with the POA over the last two weeks and have provided a comprehensive response to a range of health and safety concerns.
“The Government has announced an additional 2,500 frontline officers to help reduce violence in prisons.
“We have well-established contingencies in place to manage prisons and keep the public safe, but we are clear that this constitutes unlawful industrial action, and we will seek remedy in the courts.”
The POA has insisted the officers’ action was not illegal and was necessary because they were being forced to work in an “unlawful” situation because of a lack of health and safety in jails.
However, on Tuesday afternoon the Government won a High Court injunction against industrial action being taken, and prison officers were ordered back to work.
Granting the injunction, Mr Justice Kerr said it was a “very urgent” application with evidence of up to 80% of staff taking some sort of action in the majority of prisons.
“A number of incidents have occurred in prisons today and the situation is very concerning indeed,” the judge added.