US Treasury to reach debt limit by mid-October
"Congress should act as soon as possible to protect America's good credit by extending normal borrowing authority well before any risk of default becomes imminent," Lew said in a letter to congressional leaders.
The borrowing limit lift does not lead to increase in spending; rather, it allows the Treasury to pay expenditures Congress has previously approved, he said.
"If investors should become unwilling to loan the United States money, the United States could face an immediate cash shortfall," Lew said.
In the summer of 2011, a fight over the government's debt limit turned into a crisis that weighed heavily on markets.
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Lew said earlier this year that the debt ceiling was unlikely to be reached until September.
"People shouldn't relax—Congress should deal with this right away," he said at that time. "The uncertainty caused by putting this off is not good. The anxiety caused to the U.S. and world economy by putting this off until the last minute is not good."
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—CNBC.com contributed to this report.