Hundreds of riot police were called in tonight to take over the running of Northern Ireland's troubled prisons.
They were ordered in after prison officers refused to go back on duty in the latest stage of a bitter dispute with the authorities over security packages for warders facing terrorist threats.
Specialist units trained to deal with sectarian street disorder have been deployed, security sources disclosed.
One said: "We may have to force these people back into their cells so we definitely need public order.
"This has the potential to tie up hundreds of police officers."
All visits to the top security Maghaberry jail near Lisburn, Co Antrim, were cancelled as prison chiefs faced up to the major staffing crisis.
Magilligan jail in Co Derry and the Hydebank Young Offenders' Centre near Bangor, Co Down were also hit by the walkout.
Mr Peter Russell, director general of the Northern Ireland Prison Service, hit out at the officers who failed to return after lunch.
He said: "This action is reprehensible. It increases the potential for disruption in the prisons when there is already a volatile atmosphere.
"More than 1,600 officers manage the three regimes, and prison sources said at least 60 per cent of those on duty today quit without notice.
Some 500 officers were on duty across the three prisons; 60 per cent did not return from lunch.
Management and staff at the three institutions have been locked in a deepening row ever since officers details were allegedly stolen by the IRA during its espionage plot at Stormont last year.
Even though the British government has ploughed resources into schemes to protect the officers, union bosses have demanded more.
Loyalist and dissident republican terrorists have also carried out a series of bomb attacks on warders' homes.
Tensions also heightened when so-called dirty protests at Magilligan, when republicans smeared excrement on their cell walls, led the authorities to begin segregation between the rival paramilitary factions.
A pay dispute and plans to bring in civilian staff to help with the segregation programme had further strained relations.
Mr Finlay Spratt, chairman of the Prison Officers' Association, insisted he knew nothing in advance about the action.
But he said: "I fully support any decision they have made.
"If they are stressed out and sick they have my utmost sympathy. This was to show Government and management that they had had enough."
Police confirmed they sent officers in after being urged by the Government.
A PSNI spokesman said: "In relation to action taken by prison officers and at the invitation of the Northern Ireland Office contingency plans have been set in motion.
"The police service is contributing to the day to day running of the prisons."
But Mr Russell claimed union representatives were involved."I regard this as unofficial industrial action. It is simply not credible for the POA to suggest that this is unilateral action taken by prison officers without their approval," he said.
"Staff were warned this morning of the consequences of leaving their posts.
"It is a breach of their terms and conditions of service and as such they will not be entitled to pay."
PA