The ratings agency affirmed its "AAA/A-1+" rating for the U.K. The recently re-elected government of Prime Minister David Cameron is slated to hold a referendum on EU membership by 2017.
"The outlook revision reflects our view that the decision of the newly elected Conservative majority government to hold a referendum on the U.K.'s EU membership by 2017 represents a risk to growth prospects for the U.K.'s financial services and export sectors as well as the wider economy," S&P said in a release.
A possible EU exit also "raises questions" about the country's ability to finance its deficits and short-term external debt, S&P said. The agency noted party politics could affect economic policy more than previously anticipated.
"In our opinion, the outcome of the general election could make consensus-based policymaking more challenging," S&P said.
It could lower Britain's rating "should we conclude that departure from the EU is likely over the medium term." S&P is the only major agency to still give the country top-notch ratings.
Cameron's Conservative Party won a parliamentary majority last month, obtaining about 37 percent of the national vote. The party has a strong wing of "euroskeptics" who believe the openness of the EU economy has hurt more than helped Britain.
—Reuters contributed to this report.