The council, which has been Labour controlled since May 1994, launched an investigation into 55 pension settlements between 1989 and 1994 in the summer of 1995 (LGC, 4 August).
The inquiry by the council's external auditors Coopers & Lybrand is expected to report by Easter but pensions of the original 55 cases have already been reduced.
As a result of the inquiries the council and external auditors ran checks on payments to all its 3,000 pensions, which has thrown-up the extra 51 cases.
'We very much regret the necessity to take this action, but legally we have no option,' said leader Liz Pearce.
'We will offer every assistance to those concerned to help them ensure that their interests are fully represented.
'Where a discrepancy has been identified, consultation with the relevant individual may provide details to substantiate the existing level of payment,' she added.
The council declined to comment on the nature of the irregularities for fear of prejudicing the inquiry. It is understood that a decision on whether to claw-back past payments will be made in light of the inquiry findings.