All of the 500-plus prison officers who applied for early retirement as part of a major restructuring of the service have been given permission to leave.
The Department of Finance has approved the £30 million business case required to enable 164 officers whose applications remained outstanding to retire.
A Northern Ireland Prison Service spokesman said: "These members of staff will be allowed to leave when the necessary funding is available and it is operationally feasible for them to do so."
It is understood all the officers will likely have left within 12 months.
To date 287 officers have retired under the terms of the severance package offered in November 2011, with a further 73 scheduled to go at the end of this month. This package has cost around £40 million.
The 164 further officers whose applications have now been approved bring the total to 524.
That represents around 90% of those officers who were eligible to apply (i.e. those over 50 in November 2011).
While the total bill is set to be £70 million, the prison service insists the long-term savings will be much greater, as many of the most highly paid officers are being replaced by younger staff members on different pay scales.
The service's early retirement scheme mirrors past policing reforms in Northern Ireland, which saw many Royal Ulster Constabulary officers leave to make way for a new generation of Police Service of Northern Ireland personnel. The prison service is currently recruiting hundreds of new officers in a bid to transform the traditional make-up of the workforce.
However, prison chiefs have already expressed disappointment at the limited number of new applications from the Catholic community. The murder of prison officer David Black by dissident republicans in November last year has apparently had some impact on the recruitment process, with seven newly appointed officers having since declined to progress into the service after citing security concerns.