Credit rating agency Egan Jones downgraded the United States Thursday on concern over the sustainability of public debt in the world's biggest economy.

The U.S.'s rating was cut one notch to double A, two notches below the highest rating, down from double A plus. Egan-Jones is considered among the larger ratings firms after the big three of Standard & Poor's, Moody's Investors Service and Fitch Ratings.

Egan said the U.S.'s total debt as a percentage of gross domestic product may rise above 112% by the end of 2013, a level that will restrain the country's financial flexibility.

"Without some structural changes soon, restoring credit quality will become increasingly difficult," the company said in a statement.

Egan said there was a 1.2% probability that the U.S. would default in the next year.

S&P rates the U.S. double A plus after downgrading the country in August, while Moody's and Fitch both have a triple A rating on the U.S. While Egan has assigned the U.S. the lowest rating, all four agencies have a negative outlook for U.S. debt.