Shares of American Express Co. fell the most in six weeks Thursday after a Lehman Brothers analyst cut his 2009 profit estimate because of rising defaults.

Uncollectable credit card debts may reach 8.5% of loans next year, compared with an earlier 7.6% estimate, Lehman's Bruce Harting wrote in a research report sent to the brokerage's clients Thursday.

Amex may earn $2.55 a share next year, 12% less than his previous target, he wrote.

Mr. Harting also lowered his price target for American Express to $37 a share, from $42.

American Express may have its credit ratings downgraded after adding a bigger-than-expected $600 million to loss reserves in the second quarter as consumers struggle to repay debt.

The New York company withdrew its earnings growth forecasts in July and said it will not meet longer-term targets until the U.S. economy improves.

American Express shares closed Thursday down 5.28% from Wednesday's close, at $38.75. They have slumped 25% this year.

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