The UK is likely to lose its AAA credit rating after it voted to Leave the EU.
Moritz Kramer, chief ratings officer for S&P, said on Friday morning that the UK's AAA rating was "untenable under the circumstances".
German daily newspaper Bild quoted Kramer as saying: “If Great Britain decides for a Brexit in the EU referendum on Thursday, then the AAA credit rating would come due and would be downgraded within a short period of time."
The UK Government would be informed 24 hours before any downgrade.
The S&P is the only ratings agency of the "big three", including Moody's and Fitch, to keep the UK at the highest grade. The other two agencies downgraded the UK in 2013.
Tremors reverberated through global markets on Friday after the UK's seismic decision to leave the EU.
The Bank of England said it was monitoring developments closely and was working closely with other overseas central banks to ensure the financial stability of the UK.
FTSE Futures, which give an indication of where the FTSE 100 stock market index, were showing losses of 8 per cent on Friday morning.
The value of the pound slumped to its lowest level in more than 31 years, down to $1.33.
"We can expect the pound to continue to fall. The BoE will be monitoring the situation closely and has arranged liquidity to support the markets. Be in no doubt though, we are in for a rocky ride," said Trevor Charsley of AFEX, a global payments and risk management company.
