It is estimated that 5,000 people in Wales who are paying a spare room subsidy could - and should - have been exempt.
Under the rule, anyone who has been in their home since before 1996 and has continually claimed housing benefit does not have to pay the subsidy.
A spokesperson for the Department of Work and Pensions say they are looking at the issue carefully and will take any necessary action.
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Wayne Pivac praised Scarlets' attacking style as they secured the Guinness PRO12 title with a stunning 46-22 success against Munster.
Scarlets sealed a famous victory at the Aviva Stadium in Dublin with an emphatic 46-22 victory over Munster in the Guiness PRO12 final.