American Express, the biggest U.S. credit-card issuer by customer purchases, said its co-brand and merchant agreements with Costco Wholesale in U.S. stores will end next year.

"We were unable to reach terms that would have made economic sense for our company and shareholders," Amex Chief Executive Kenneth I. Chenault said Thursday in a news release. "Instead, we will focus on opportunities in other parts of our business where we see significant potential for growth and attractive returns over the moderate to long term."

Amex is under pressure to keep partnerships as competition among banks and payment networks for co-brand deals intensifies and merchants seek better terms. The New York lender's exclusive role as the credit-card provider for Costco is worth at least 15 cents a share of this year's profit, according to analysts estimates. Costco said last year it was replacing Amex in Canada.

American Express dropped 1.2% to $85 at 7:38 a.m. in New York. The shares fell 0.9% on Nov. 6 when Bloomberg News reported that Costco, the largest U.S. warehouse-club chain, was considering replacing Amex as its credit card issuer in the U.S.

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