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

The stock dropped 6.6 percent to $80.33 at 9:46 a.m. in New York, the largest drop since August 2011 and the biggest decliner in the 30-company Dow Jones Industrial Average.

"We were unable to reach terms that would have made economic sense for our company and shareholders," Amex Chief Executive Officer Kenneth I. Chenault said Thursday in a statement. "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-based 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.

Amex shares fell 0.9 percent 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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