American Express Co., the largest credit-card company by purchases, may be cut by Standard & Poor’s on the prospect more borrowers will default.

The company was put on “CreditWatch” for a downgrade based on “escalating credit quality deterioration,” the ratings firm said yesterday in a statement on American Express.

The lender dropped $1.02, or 7.24 percent, to $13.07 yesterday in New York Stock Exchange composite trading.