Thousands of prison officers in England and Wales are striking in a protest over pay, the association representing them has said.
Thousands of prison officers in England and Wales are striking for at least 24 hours in a protest over pay.
The Prison Officers' Association says the strike, which started at 0700 BST, will go on for a minimum of 24 hours.
The Prison Officers' Association's strike, which started at 0700 BST, comes after it pulled out of a no-strike agreement with government.
It says it is staging the strike - which follows a ballot of members - after pulling out of a no-strike agreement with the government.
Officers in Leeds, Liverpool, Kent, Birmingham, East Anglia and London were among those joining the strike.
The government said the officers were acting illegally.
The government called the strike illegal while Justice Secretary Jack Straw called it "wholly unjustifiable".
Mr Straw said officials had approached the POA on Tuesday to organise a meeting.
The strike is the first national walkout in the association's 68-year history.
The strike is the first national walkout in the association's 68-year history.
The Prison Officers' Association (POA), which has 28,000 members, said up to 90% of its members who had been due on duty had joined the strike.
The Prison Officers' Association (POA), which has 28,000 members, said up to 90% of its members who had been due on duty had joined the strike.
POA representatives at Wormwood Scrubs prison, in west London, said the 1,300 prisoners were being looked after by eight governors.
POA representatives at Wormwood Scrubs prison, in west London, said the 1,300 prisoners were being looked after by eight governors.
The Prison Governors' Association said the walkout had been "widespread and unprecedented" and that there was "lockdown" - where prisoners are confined to their cells - at most prisons.
Prison safety
Prison safety
A spokeswoman for the Ministry of Justice said the walkout was a breach of the Industrial Relations Act.
The Ministry of Justice said the walkout was a breach of the Industrial Relations Act and that it would be seeking advice on "our next step".
"We will be seeking advice on our next step," the spokeswoman added.
Mr Straw said in a statement that contingency plans were in place to maintain security in prisons where officers had gone on strike.
At my request yesterday, my office asked for a meeting with senior officers of the POA Jack Straw
"Our first concern in this situation is to protect the public," he added.
"Ministers have met with the POA on a regular basis in recent weeks and were due to meet next month.
"Indeed, at my request yesterday, my office asked for a meeting with senior officers of the POA."
The BBC's Danny Shaw said ministers were considering court action.
The POA said it "fully expected" the Ministry of Justice to seek an injunction to stop the strike action.
The POA said it "fully expected" the Ministry of Justice to seek an injunction to stop the strike action.
Prisoners are being kept in their cells at the moment.
Prisoners are being kept in their cells at the moment.
It is believed police may have to be drafted in to ensure the safety of inmates.
It is believed police may have to be drafted in to ensure the safety of inmates.
The walkout follows a POA national ballot of members two weeks ago.
The walkout follows a POA national ballot of members two weeks ago.
Surely we should be treated as well, if not better, than the prisoners we look after Colin MosesPOA
Some 87% of the prison officers who voted endorsed industrial action "up to and including strike".
Some 87% of the prison officers who voted endorsed industrial action "up to and including strike".
Pay row
An independent pay review body recommended a 2.5% pay rise but the government staged that rise as 1.5% in April and a further 1% in November.
An independent pay review body recommended a 2.5% pay rise but the government staged that rise as 1.5% in April and a further 1% in November.
Surely we should be treated as well, if not better, than the prisoners we look after Colin MosesPOA
The POA says that, due to inflation, this reduces the value of the award, making it a below-inflation pay increase.
The POA says that, due to inflation, this reduces the value of the award, making it a below-inflation pay increase.
Prison Officers' Association (POA) general secretary Brian Caton told BBC News: "We told government and indeed the Prison Service of the result of that ballot and we asked for meetings.
Prison Officers' Association (POA) general secretary Brian Caton told BBC News: "We told government and indeed the Prison Service of the result of that ballot and we asked for meetings.
"None of those meetings have happened and that has caused our membership to call for strike action which we have taken today."
"None of those meetings have happened and that has caused our membership to call for strike action which we have taken today."
Chairman Colin Moses said the decision to strike had been taken "after two years of frustration and two years of below-inflation pay awards".
Chairman Colin Moses said the decision to strike had been taken "after two years of frustration and two years of below-inflation pay awards".
"Surely we should be treated as well, if not better, than the prisoners we look after."
"Surely we should be treated as well, if not better, than the prisoners we look after."
Injunction 'likely'
'Cross and fed-up
The Prison Governors' Association has described the strike action as "widespread but patchy".
Prison Governors' Association chairman Charles Bushell told BBC News: "The next big issue will be at lunchtime.
At some jails, a large percentage of officers had gone out on strike, chairman Charles Bushell told BBC News.
"Each individual governor is having to find the best way he or she or can to ensure that people get something to eat and will do so again this evening."
There would also be strong action at jails in Liverpool, Leeds and Manchester, he added.
The majority of prisoners would become "cross and fed-up" but would "control themselves", he said.
He said he was confident the Prison Service would take the POA to court on Tuesday to get an injunction stopping the strike and preventing more.
"Equally, we know that there is a minority - quite a sizeable minority in some places - who will get cross and will find it difficult."
The governors would try to minimise disruption to prisoners, Mr Bushell added.
Prisoners would not be able to see visitors and very few prisoners expected in court on Wednesday would be taken there, he added.
"Where they are going to suffer is that there aren't going to be visits today," he told BBC News.
"They're not going to be able to get out of their cells and, furthermore, courts which were expecting prisoners to be produced to them are unlikely to have those prisoners brought to court in many cases."
The Northern Ireland Prison Service said its prisons were unaffected by the action.
The Northern Ireland Prison Service said its prisons were unaffected by the action.
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