The switch from a tax on profits would expose the huge U.K. sales of global internet companies to tax authorities.
Negative rating actions on sovereign debt "are likely to continue to outnumber positive actions over the coming 12 months," S&P says.
The United Kingdom thrived as an EU member, and leaving will destabilize the economy and the country's finances, says S&P.
The "Brexit" will deter investment in the U.K.'s economy and put its financial sector at a competitive disadvantage, S&P predicts.
"The balance of evidence points to notable downward economic risks to the U.K. economy" if the country leaves the EU, the Fund warns.
The change may ease criticism over Facebook's overseas tax arrangements but won't necessarily mean a tax windfall for the U.K. government.
