SINN Féin Foyle MLA Karen Mullan has called on the Department of Communities to reimburse ESA claimants “as matter of urgency”.
The call comes after revelations that as many as 8,000 ESA claimants have been underpaid by £3,000 on average.
The Department plans on reimbursing claimants by April 2019.
Karen Mullan said: “Sinn Féin first raised this issue with the Department several weeks ago, and called on them to reimburse claimants as a matter of urgency.
“I welcome that the Department has acknowledged the extent of this problem, and have clarified that all affected claimants will be reimbursed.
“However, it highlights a very serious breakdown in a system which must always rigorously check each claim to ensure full entitlement of financial support.
“Those denied this benefit were among the most in need of financial support at the time of their claim.
“Having been denied their full benefit entitlements for a number of years, it is unacceptable that reimbursement will not be finally made until April 2019.
“Despite the assurance offered by the Department, I urge all those in Derry who believe they may have been denied this benefit to seek independent advice, or contact the Department asking for a review of their claim,” added the Foyle MLA.