Standard & Poor's said its president is stepping down, capping two weeks of controversy following the rating agency's downgrade of U.S. government debt on Aug. 5 that sparked a row with the Treasury.

S&P's parent McGraw-Hill Companies Inc , said on Tuesday Deven Sharma, who has served as S&P president since 2007, will be succeeded on Sept. 12 by Citibank Chief Operating Officer Douglas Peterson.

"S&P will continue to produce ratings that are comparable, forward looking and transparent," McGraw-Hill said in a statement, adding that Sharma would work on a strategic portfolio review for the group until leaving at year-end.

The one-notch downgrade of U.S. government debt from AAA, which has not been matched by other major rating agencies, led to the biggest sell-off in global stock markets in three years and was criticised by Treasury officials and the administration of President Barack Obama over some of the methodology used by S&P.

The U.S. Justice Department is also investigating the ratings agency over its actions in assigning high ratings to complex mortgage securities leading up to the 2008-2009 financial crisis, a source familiar with the matter told Reuters last week.

S&P management has been criticized for its handling of the downgrade, although some advocates said it was acting responsibly in light of mounting debt and Washington's political inability to devise a long-term solution to the issue.

The Financial Times, which first reported Sharma's resignation, quoted unnamed sources on Tuesday saying his departure was unrelated to the downgrade or the Justice Department investigation.