Fitch is the second credit rating agency to knock Britain down a notch, joining Moody's.
The third major credit rating firm, Standard & Poor's, has listed Britain as having a "negative outlook," The Daily Telegraph reported Saturday.
"The weak growth performance of the U.K. economy in recent years, partly due to private and public sector deleveraging and the eurozone crisis," contributed to the downgrade, Fitch said.
Britain's rating was "stable," which implies it will not be subjected to another review soon, the announcement said. Fitch said it chose a "stable" outlook rating because of Britain's "continued policy commitment to reducing the underlying budget deficit."