A British court ordered thousands of prison officers back to work on Tuesday, saying there had been incidents in prisons during their industrial action and calling the situation "very concerning".
Union leaders had called the 24-hour work stoppage saying the prison system was "in meltdown" but the government said the action was "unlawful" because guards are not allowed to go on strike.
The protest caused disruption in courts -- halting the high-profile murder trial in London of the man accused of killing MP Jo Cox -- after talks between union bosses and the government broke down.
Britain's largest prisons union, the Prison Officers Association (POA), directed its members to take part in the action over what it described as the "volatile and dangerous state of prisons".
But Justice Minister Liz Truss told MPs in the House of Commons: "The union's position is unnecessary and unlawful and it will make the situation in our prisons more dangerous."
The High Court later ruled against the union.
"A number of incidents have occurred in prisons today and the situation is very concerning indeed," read the ruling, which ordered the officers to return to work "forthwith".
Joe Simpson, assistant general secretary of the POA, said 10,000 members around the country were participating in the stoppage, but insisted it was "not a strike".
The union has around 30,000 members and there are around 85,000 prisoners in Britain.
Incidents of violence, self harm and deaths in custody have been "steadily rising", reaching "totally unacceptable" levels, he told AFP.
The protest comes after a series of recent incidents at British prisons including a riot involving up to 200 inmates, the fatal stabbing of an inmate, and the escape of two other prisoners.
The POA blamed the problems on "chronic staff shortages and impoverished regimes".
"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," the union said in a statement.
Standing outside Pentonville prison in London, union member Dave Todd, said: "Our main concerns today are rises in violence... It's just not manageable any more".
Union members "have been suffering violence, physical violence, verbal violence, levelled at them".
Contacted by AFP following the court ruling, the POA said it was not changing its instruction to protesting prison officers.
A justice ministry spokesman said the government had addressed union concerns, including with an additional 2,500 frontline officers. afp