The reassessment issued late yesterday recast S&P's view on the UK's credit rating to "stable" from "negative." The influential agency also reaffirmed its top-notch "AAA" credit rating.
But S&P warned that the rating could be imperiled if Britain leaves the European Union, a move the country's Conservative Party has pledged to put to a referendum in 2017 if it wins the next general election.
Defecting from the European Union would be a significant blow because the UK has considerable trade ties to the region and could lose inbound investments, S&P said.
S&P is optimistic about worker productivity and wages increasing. The agency is also encouraged with progress being made on the UK's government budget deficits.
After adding it all up, S&P concluded there is only a one-in-three chance of a credit downgrade during the next two years.