MasterCard Inc., the world's second- largest electronic payments network, will lose more than half of a $59 billion portfolio of U.S. debit-card users after JPMorgan Chase & Co. decided to shift more business to Visa Inc., two people familiar with the matter said.

MasterCard, which collects fees to process payments between banks and merchants, held the portfolio since 2005, said the people, who declined to be identified because the switch hasn't been officially announced. The customers had checking accounts at Seattle-based Washington Mutual Inc. until JPMorgan bought assets of the failed lender last year, the people said. Bidding for the accounts began in October, the people said.

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