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.

S&P has previously said its AAA rating for Britain reflected its assumption that the referendum would deliver a vote to remain in the EU.