400 prisoners are locked up in their cells 23 hours a day because they are in fear of being killed in jail.

A growing gang culture in prisons has forced authorities to take drastic measures to ensure the safety of those under threat.

The director general of the Irish Prison Service, Brian Purcell, admitted yesterday there were now more than 900 prisoners under protection in the nation's jails.

He acknowledged there was a problem with gang culture in society that was also being reflected behind prison walls.

But he denied claims made by delegates at the annual conference of the Prison Officers Association in Kilkenny that the gangs were taking control of the system.

He said prisoners had to be kept under protection because of threats to their security.

This was not an indication the prisons were unsafe but a measure of what the prison service was doing to ensure inmates were not harmed.

Mr Purcell said between 340 and 370 prisoners were kept in their cells for 23 hours a day and then allowed to exercise on their own for the other hour.

"If we didn't have the lock-ups, there would be an increasing risk," he said.

But the vast majority of the protected prisoners was subject to a regime that was similar to that experienced by most of the prison population, he said.

He revealed members of 10 to 12 different gangs were held in the prison system.

This meant that the different factions had to be kept apart, as far as possible.

Association president Stephen Delaney said there were 60 protected prisoners in Dublin's Mountjoy jail, including members of nine different factions, and this created significant problems for staff there.

Overall, there are about 4,500 people in custody and 5,500 in the system, including those on temporary release, according to Mr Purcell.

Projections for the next three to four years push the figures up to 7,000 in the system.

He said there were additional prison spaces due to come on stream in Mountjoy and Dochas next year, and the service was also looking at other options.

But he said no definite decisions on future prison building could be made until the Thornton Hall project review committee made its recommendations, which are due to be delivered to Justice Minister Alan Shatter by July 1.

Overcrowding

He said Thornton Hall had been a key plank in the strategy to deal with overcrowding in the past, and that the committee had to come up with some other solutions to that problem.

Mr Purcell said there were about 150 assaults by prisoners on staff in a year.

"Given the type of people we deal with, this is not a huge number but every assault must be deplored," he said.

He noted there were also 760 assaults by prisoners on other prisoners in a year, slightly over two a day. "We have 1.4 million prisoner bed nights in our jails on an annual basis, so two assaults a day is a relatively low number," he said