S&P cut the long-term US rating by one notch to AA+ with a negative outlook, citing concerns about budget deficits.
The agency said the deficit reduction plan passed by the US Congress on Tuesday did not go far enough.
Correspondents say a downgrade could further erode global investors’ confidence in the world’s largest economy, which is already struggling with huge debts, unemployment of 9.1%, and beset by fears of a possible double-dip recession.
S&P said in its report issued late on Friday: “The downgrade reflects our opinion that the fiscal consolidation plan that Congress and the administration recently agreed to falls short of what, in our view, would be necessary to stabilise the government’s medium-term debt dynamics.
“More broadly, the downgrade reflects our view that the effectiveness, stability, and predictability of American policymaking and political institutions have weakened at a time of ongoing fiscal and economic challenges.”
The agency said it might lower the US long-term rating another notch to AA within the next two years if its deficit reduction measures were deemed inadequate.