Prison officers protesting over "unprecedented levels of violence" in jails
Thousands of members of the Prison Officers' Association (POA) have returned to work after staging a walkout over safety and overcrowding worries at UK jails.
It comes after the chief inspector of prisons warned about the potential for a "complete breakdown" in order and discipline at HMP Bedford.
The Ministry of Justice (MoJ) said the protests across Wales and England are illegal and it is seeking a court injunction to stop them.
Notices handed to prison officers protesting outside HMP Bedford warned that the industrial action was "unlawful" and in breach of their employment contracts.
The letters, issued by the prison's governor Helen Clayton-Hoar, said: "I am therefore giving you a direct order to return to work.
"I must also advise you that if you do not return to work and ignore this order, that deductions will be made to your pay and you may be subject to disciplinary procedures."
The POA expects at least 5,000 prison officers across England and Wales to take part in the protests, which started at 7am.
Steve Gillan, general secretary at the POA, said there was a "rising epidemic" of violence and drug taking in prisons.
He told Sky News: "We can't just keep turning a blind eye to the broken limbs, the smashed eye sockets and broken jaws of our members. They're people as well.
"Everybody has a right to go to work not in fear for their health and safety."
The POA said there had been 116 assaults on staff at HMP Bedford in the last six months.
A watchdog found that inmates have effectively taken control at the violent, overcrowded and vermin-infested jail.
"The unprecedented levels of violence, and failure of this government and employer to provide safe prisons has been headline news for some considerable time," a POA spokesman said.
"The rise in violence against staff in prisons is laid firmly at the feet of government and Her Majesty's Prison and Probation Service, who have overseen the demise of the prison service over the last eight years."
Mr Gillan said he did not expect prisoners to riot and there was "minimum cover" in place to ensure the safety of those serving jail sentences.
Sky News home affairs correspondent Mark White said prisoners would be locked in their cells during the protests.
The POA is banned from taking strike action but the union argues the protests are lawful under health and safety laws, White said.
Prisons minister Rory Stewart, who has pledged to resign if his campaign to tackle drugs and violence in jails is a failure, branded the POA "irresponsible" and said the protests were illegal.
He 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 illegal 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."