The regulator alleged Tuesday that the retailers' "Buy Now, Pay Later" promotions, often resulted in customers paying more than advertised.
The bureau said additional fees were "buried" in the "fine print," and resulted in customers who chose the deferred-payment option paying more than those who paid for their purchases up front.
The additional costs ranged from processing or administrative fees, delivery fees and taxes.
The allegations have not been proven in court.
In a case filed in Ontario Superior Court, the Competition Bureau wants Leon's and The Brick to refund all customers who paid the additional up-front fees and administrative penalties.
"Canadian consumers must receive clear and accurate information about what must be paid at the time of purchase, and what the actual cost of a particular item is if they use a deferred payment option," Competition Commissioner John Pecman said in a statement.
"Retailers cannot hide details of additional fees in lengthy disclaimers."
The retailers were not immediately available for comment.
Last March, Leon's acquired rival, The Brick.
Leon's has 76 stores with locations in every province except British Columbia, while The Brick has 230 stores operating under the banners, The Brick, United Furniture Warehouse, The Brick Mattress Store and Urban Brick banners.
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