The Government has won a High Court injunction to force up to 10,000 protesting prison officers to return to work.
Thousands of staff stopped working on Tuesday over health and safety fears, amid claims the service is in "meltdown".
It is illegal for prison officers in England and Wales to strike and the Government submitted an emergency application for an injunction to "restrain" the Prison Officers Association (POA) from "inducing any form of industrial action".
The judge, Mr Justice Kerr, said: "A number of incidents have occurred in prisons today and the situation is very concerning indeed."
His order will have immediate effect with prison officers expected to end the protest "forthwith".
The POA directed all of its members to effectively go on strike for a day after talks with ministers broke down.
It has forced a delay to court cases such as the murder trial of MP Jo Cox.
Jurors at the Old Bailey were told defendant Thomas Mair, who is accused of killing the West Yorkshire MP on 16 June, was unable to attend court because of the action.
Sky's Mark White, at the hearing, said proceedings had been adjourned until Wednesday.
"Those prisoners on remand, who are facing court cases, for the most part cannot attend court," he said.
"There are not the staff or resources to be able to process them through the prison and out to the courts. There is a certain amount of chaos."
Official figures show that in the year to June, the number of attacks on staff rose by 43% to 5,954, with 697 of them recorded as serious.
A POA spokesman said: "The POA has consistently raised the volatile and dangerous state of prisons as chronic staff shortages and impoverished regimes has resulted in staff no longer being safe, a lack of discipline and prisoners taking control of areas.
"The continued surge in violence and unprecedented levels of suicide and acts of self-harm, coupled with the recent murder and escapes, demonstrate that the service is in meltdown."
Staff taking part in Tuesday's protest have been providing emergency cover only, ensuring inmates are fed and given any medication.
Prisoners have not been allowed any visitors or phone calls and have remained locked in their cells.
Dave Todd, a POA representative who served in the Army, said: "I felt more vulnerable walking the landings in prisons than I did on the streets of Northern Ireland.
"There's so many weapons within prisons, there's so many prisoners out at once. They can slash, stab and kick, beat, take you hostage if they so wish."
Governors had earlier warned striking staff against taking part in the protest.
In a letter to staff at Feltham Young Offenders Institution, obtained by Sky News, the governor urged them to "report for duty", warning: "Your action is a breach of contract and may result in you not being paid."
The action was launched mid-negotiations between Justice Secretary Liz Truss and the POA over safety in jails.
"These talks were due to continue this morning," she told MPs. "Instead the POA failed to respond to our proposals and called this unlawful action without giving any notice."
Her plans for reform include putting an extra 2,500 guards in prisons and introducing no fly zones over jails so that drones cannot drop contraband items.