Ratings agency Standard and Poor's said Britain's top-notch "AAA" credit rating is no longer tenable after voters opted to leave the European Union, the Financial Times reported on Friday.
"We think that a AAA-rating is untenable under the circumstances," Moritz Kraemer, chief ratings officer for S&P, told the FT.
Rival ratings agencies Fitch and Moody's have already stripped Britain of their AAA ratings, long before the referendum campaign began.
Moody's said on Friday that Britain's vote to leave the EU was "credit negative" for Britain's sovereign ratings and for other issuers in the country.
"This outcome heralds a prolonged period of policy uncertainty that will weigh on the UK's economic and financial performance," it said in a statement.
"Heightened uncertainty will likely dent investment flows and confidence, weighing on the UK's growth prospects - a credit negative for the U.K. sovereign and other U.K. debt issuers."
Moody's rates the U.K. one notch below triple-A with a stable outlook.