Thousands of British prison officers walked out of their jobs on Friday, gathering outside jails to protest against what their union called unprecedented levels of violence and the failure of the government to make prisons safe.
The government called on them to return to work, describing the protest as unlawful and the Prisons Officers' Association (POA) union said it had been threatened with court action.
The state of prisons in Britain has made headlines in recent years. Inspectors warned on Thursday that inmates of Bedford jail in central England did not follow rules and were using drugs. Last month, the government took over the running of another prison for similar reasons.
Prisons Minister Rory Stewart said officers should return to their duty stations in line with their obligations to the law and the prison service.
"These prison officers are putting their fellow staff and inmates at risk," he said in a statement.
The POA said the walkout would last until its general secretary instructed them to do otherwise.
The union, which has 28,000 members, wants the government to provide safe prisons, meet its demands for improving personal protective equipment and reduce levels of violence and overcrowding in prisons.
"The violence has been brought by a reduction in staffing levels," said Blessing Uwede, POA representative, at HMP Wormwood Scrubs in west London, where about 35 staff had walked out.
The government said it was addressing POA concerns.
"Yesterday we doubled the prison sentence for anyone who assaults prison officers," it said in Stewart's statement. "We've also increased pay, provided tools such as body-worn cameras to increase security on the landings, and are investing 40 million pounds ($52 million) to improve the estate and tackle the drugs problem which is fuelling much of the violence.
There were now also 3,500 new prison officers to help ease the burden, it added.