The U.S. Treasury Department said Standard and Poor’s (S&P) downgrade of the U.S. credit rating lacks justifiable grounds.

Acting Assistant Secretary for Economic Policy John Bellows questioned the creditability of S&P's decision on Saturday in a post on the Treasury Department Web site, claiming that the credit rating agency made a "$2 trillion mistake" but still choose to downgrade the U.S. credit rating, even after Treasury pointed out the error, by changing the rationale for its decision.

Beers, who is the head of sovereign ratings at S&P, said that the so-called mistake in estimating the size of debt was not the key issue behind the downgrade.

He said the key factor behind the downgrade came from the debt-ceiling debate, the "unpredictability" in policy-making shown by the White House and the Republican Party.