Fitch's Colquhoun also reiterated that the rating agency would place the U.S. sovereign rating on watch negative if Congress did not raise the federal government's borrowing ceiling by August 2, and said if the U.S. government misses an August 15 coupon payment, then Fitch would place the rating on restricted default.
But it added it believed it was very likely that the debt ceiling would be raised and default would be avoided.
Fitch had made similar comments earlier this month and Moody's and S&P have issued warnings along the same lines. But Fitch was the first major ratings agency to say U.S. Treasury securities could be downgraded, even for a short period.
U.S. lawmakers working to rein in rising debt said on Monday they will have to make substantial progress this week to ensure the country retains its top-notch credit rating.
