Martin Lewis has warned that more than 200,000 women could be owed an estimated £13,000 following a government error.
The Money Saving Expert raised the issue on his ITV Martin Lewis Money Show tonight.
Martin explained who could be affected and how they could reclaim the cash they were owed.
Martin said that married women and widows may have been had their pension significantly underpaid.
The women affected are those who hit state pension age before April 6, 2016 and had a pension that was less than 60% of their husbands.
Martin said he had previously raised the issue on his show, but since then the Department for Work and Pensions had said they would be refunding people automatically.
The DWP said they would be now backdating payments and offering top-ups automatically, with people no longer needing to claim separately.
The women affected are those whose husband turned 65 on or after March 17, 2008 or those women who had no uplift when their husband died.
But Martin warned that some women would still have to reclaim themselves.
Martin said that women who would need to claim for themselves were those whose husband turned 65 before March 2008 and you're being underpaid by 60%.
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Women who got divorced after retirement and had a pension less than their ex-husbands would also have to claim for themselves.
Martin pointed to several online guides on the show and said that women should read up on the issue further, but that they could get thousands back if they claimed successfully.
*The Martin Lewis Money Show continues at 8:30pm on ITV on Thursday
